Update BGP Allow List test case

[abdosi]

Fix sonic-net/sonic-buildimage#6572

What/Why I did:

• Updated test case to align with [bgpcfgd]: Support default action for "Allow prefix" feature sonic-buildimage#6370 which made allow list action dynamic
• Added test case to cover Fix Allow prefix Delete case sonic-buildimage#6671
• Remove fixed value of Drop Community and read from /etc/sonic/constants.yml file.
• Organized the case as three test case one to verify default pre/post-config behavior when no prefix list there on device and
  then test case to verify with prefix list programmed with action as permit and deny

How I verify:

201911 image is passing with this change.

adosi@624e49377782:/var/src/Networking-acs-sonic-mgmt/tests$ py.test --inventory "../ansible/str,../ansible/veos" --host-pattern str-xxxx-on-2 --module-path "../ansible/library/" --testbed vms13-5-t1-lag --testbed_file "../ansible/testbed.csv" --log-cli-level info -ra -vvv "bgp/test_bgp_allow_list.py" --skip_sanity

[lgtm-com]

This pull request introduces 1 alert when merging 5e82113 into f19c4c8 - view on LGTM.com

new alerts:

• 1 for Unused import

[yxieca]

@abdosi please fix the lgtm issue

[abdosi]

@abdosi please fix the lgtm issue

@yxieca thanks !! done

[lguohan]

tested on master.

[lguohan]

can you add this test to kvmtest.sh t1-lag test suite, i'd like to have this test running per pr.

[abdosi]

can you add this test to kvmtest.sh t1-lag test suite, i'd like to have this test running per pr.

can you add this test to kvmtest.sh t1-lag test suite, i'd like to have this test running per pr.

@lguohan Done

[lguohan]

i do not quite understand why it fails, i tested it locally with master image. @abdosi , is it expected?

@lguohan Once this merged sonic-net/sonic-buildimage#6671 it will pass. It is new case I added today

FAILED bgp/test_bgp_allow_list.py::TestBGPAllowListBase::test_default_allow_list_postconfig

[abdosi]

@wangxin can you please approve this