[pytest][qos][config] Added pytests for "config qos reload" commands

[smaheshm]

- What I did
Added pytests for "config qos reload" commands for single and multi ASIC platforms.

NOTE: The origin PR #1266 was reverted due to a failure in sonic-buildimage unit tests,
which was due to an un-implemented API. This is now fixed in
PR sonic-net/sonic-buildimage#6309 (sonic-buildimage)

- How I did it
Since 'config qos reload' command invokdes'sonic-cfggen' process, add a "--dry-run" option to 'config qos reload' command so that it passes the right parameters to 'sonic-cfggen' to not 'read and write' to config DB in unit test env. Also added '--json-data' option that gives flexibility to test variable parameters like tiers and switch roles.

- How to verify it

Unit test.

======================================
samaddik@83f02bcc27aa:/sonic/src/sonic-utilities/tests$ pytest-3  -s -k reload
============================= test session starts ==============================
platform linux -- Python 3.7.3, pytest-3.10.1, py-1.7.0, pluggy-0.8.0
rootdir: /sonic/src/sonic-utilities/tests, inifile: pytest.ini
plugins: cov-2.6.0
collected 337 items / 335 deselected

config_test.py SETUP
Saving output in /tmp/qos_config_output.json
0
Running command: /usr/local/bin/sonic-cfggen  --additional-data '{"DEVICE_METADATA": {"localhost": {}}}' -t /sonic/src/sonic-utilities/tests/qos_config_input/buffers.json.j2,/tmp/qos_config_output.json -t /sonic/src/sonic-utilities/tests/qos_config_input/qos.json.j2,/tmp/qos_config_output.json -y /sonic/src/sonic-utilities/tests/qos_config_input/sonic_version.yml

.TEARDOWN
SETUP
Saving output in /tmp/qos_config_output.json<0,1,2..>
0
Running command: /usr/local/bin/sonic-cfggen -n asic0 --additional-data '{"DEVICE_METADATA": {"localhost": {}}}' -t /sonic/src/sonic-utilities/tests/qos_config_input/0/buffers.json.j2,/tmp/qos_config_output.json0 -t /sonic/src/sonic-utilities/tests/qos_config_input/0/qos.json.j2,/tmp/qos_config_output.json0 -y /sonic/src/sonic-utilities/tests/qos_config_input/sonic_version.yml
Running command: /usr/local/bin/sonic-cfggen -n asic1 --additional-data '{"DEVICE_METADATA": {"localhost": {}}}' -t /sonic/src/sonic-utilities/tests/qos_config_input/1/buffers.json.j2,/tmp/qos_config_output.json1 -t /sonic/src/sonic-utilities/tests/qos_config_input/1/qos.json.j2,/tmp/qos_config_output.json1 -y /sonic/src/sonic-utilities/tests/qos_config_input/sonic_version.yml

.TEARDOWN


=================== 2 passed, 335 deselected in 1.51 seconds ===================

- 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)

[smaheshm]

retest this please

[smaheshm]

@judyjoseph This was already approved and committed but reverted due to vs-image test failure. Recreated a new PR.