[202012][vlan] Refresh dhcpv6_relay config while adding/deleting a vlan (#2660)

[yaqiangz]

Why I did

Cherry-pick and resolve conflicts of this PR: #2660
Currently, add/del a vlan doesn't change related dhcpv6_relay config, which is incorrect.

How I did it

1. Add dhcp_relay table init entry while adding vlan
2. Delete dhcp_relay related config while deleting vlan
3. Add unitest

How to verify it

1. By unitest

===================================================================================================================== test session starts ======================================================================================================================
platform linux -- Python 3.9.2, pytest-6.0.2, py-1.10.0, pluggy-0.13.0 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /sonic/src/sonic-utilities, configfile: pytest.ini
plugins: pyfakefs-5.0.0, cov-2.10.1
collected 2171 items  

tests/vlan_test.py::TestVlan::test_show_vlan PASSED                                                                                                                                                                                                      [ 71%]
tests/vlan_test.py::TestVlan::test_show_vlan_brief PASSED                                                                                                                                                                                                [ 72%]
tests/vlan_test.py::TestVlan::test_show_vlan_brief_verbose PASSED                                                                                                                                                                                        [ 72%]
tests/vlan_test.py::TestVlan::test_show_vlan_brief_in_alias_mode PASSED                                                                                                                                                                                  [ 72%]
tests/vlan_test.py::TestVlan::test_show_vlan_brief_explicit_proxy_arp_disable PASSED                                                                                                                                                                     [ 72%]
tests/vlan_test.py::TestVlan::test_show_vlan_config PASSED                                                                                                                                                                                               [ 72%]
tests/vlan_test.py::TestVlan::test_show_vlan_config_in_alias_mode PASSED                                                                                                                                                                                 [ 72%]
tests/vlan_test.py::TestVlan::test_config_vlan_add_vlan_with_invalid_vlanid PASSED                                                                                                                                                                       [ 72%]
tests/vlan_test.py::TestVlan::test_config_vlan_add_vlan_with_exist_vlanid PASSED                                                                                                                                                                         [ 72%]
tests/vlan_test.py::TestVlan::test_config_vlan_del_vlan_with_invalid_vlanid PASSED                                                                                                                                                                       [ 72%]
tests/vlan_test.py::TestVlan::test_config_vlan_del_vlan_with_nonexist_vlanid PASSED                                                                                                                                                                      [ 72%]
tests/vlan_test.py::TestVlan::test_config_vlan_add_member_with_invalid_vlanid PASSED                                                                                                                                                                     [ 72%]
tests/vlan_test.py::TestVlan::test_config_vlan_add_member_with_nonexist_vlanid PASSED                                                                                                                                                                    [ 72%]
tests/vlan_test.py::TestVlan::test_config_vlan_add_exist_port_member PASSED                                                                                                                                                                              [ 72%]
tests/vlan_test.py::TestVlan::test_config_vlan_add_nonexist_port_member PASSED                                                                                                                                                                           [ 72%]
tests/vlan_test.py::TestVlan::test_config_vlan_add_nonexist_portchannel_member PASSED                                                                                                                                                                    [ 72%]
tests/vlan_test.py::TestVlan::test_config_vlan_add_portchannel_member PASSED                                                                                                                                                                             [ 72%]
tests/vlan_test.py::TestVlan::test_config_vlan_add_rif_portchannel_member PASSED                                                                                                                                                                         [ 72%]
tests/vlan_test.py::TestVlan::test_config_vlan_with_vxlanmap_del_vlan PASSED                                                                                                                                                                             [ 72%]
tests/vlan_test.py::TestVlan::test_config_vlan_del_vlan PASSED                                                                                                                                                                                           [ 72%]
tests/vlan_test.py::TestVlan::test_config_vlan_del_nonexist_vlan_member PASSED                                                                                                                                                                           [ 72%]
tests/vlan_test.py::TestVlan::test_config_add_del_vlan_and_vlan_member PASSED                                                                                                                                                                            [ 72%]
tests/vlan_test.py::TestVlan::test_config_add_del_vlan_and_vlan_member_in_alias_mode PASSED                                                                                                                                                              [ 73%]
tests/vlan_test.py::TestVlan::test_config_vlan_proxy_arp_with_nonexist_vlan_intf_table PASSED                                                                                                                                                            [ 73%]
tests/vlan_test.py::TestVlan::test_config_vlan_proxy_arp_with_nonexist_vlan_intf PASSED                                                                                                                                                                  [ 73%]
tests/vlan_test.py::TestVlan::test_config_vlan_proxy_arp_enable PASSED                                                                                                                                                                                   [ 73%]
tests/vlan_test.py::TestVlan::test_config_vlan_proxy_arp_disable PASSED                                                                                                                                                                                  [ 73%]
tests/vlan_test.py::TestVlan::test_config_2_untagged_vlan_on_same_interface PASSED                                                                                                                                                                       [ 73%]
tests/vlan_test.py::TestVlan::test_config_set_router_port_on_member_interface PASSED                                                                                                                                                                     [ 73%]
tests/vlan_test.py::TestVlan::test_config_vlan_add_member_of_portchannel PASSED                                                                                                                                                                          [ 73%]
tests/vlan_test.py::TestVlan::test_config_add_del_vlan_dhcp_relay[ipv4] PASSED                                                                                                                                                                           [ 73%]
tests/vlan_test.py::TestVlan::test_config_add_del_vlan_dhcp_relay[ipv6] PASSED                                                                                                                                                                           [ 73%]


----------- coverage: platform linux, python 3.9.2-final-0 -----------
Name                                                Stmts   Miss Branch BrPart  Cover
-------------------------------------------------------------------------------------
utilities_common/dhcp_relay_util.py                    15      2      2      1    82%
-------------------------------------------------------------------------------------
TOTAL                                               39535  11393  14040   2014    68%

2. install whl and run cli

admin@bjw-can-720dt-2:~$ sonic-db-cli CONFIG_DB hgetall "DHCP_RELAY|Vlan1001"
{}
admin@bjw-can-720dt-2:~$ sudo config vlan add 1001
admin@bjw-can-720dt-2:~$ sonic-db-cli CONFIG_DB hgetall "DHCP_RELAY|Vlan1001"
{'vlanid': '1001'}
admin@bjw-can-720dt-2:~$ sudo config vlan del 1001
Restarting DHCP relay service...
admin@bjw-can-720dt-2:~$ sonic-db-cli CONFIG_DB hgetall "DHCP_RELAY|Vlan1001"
{}

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)