[pfcwd] Add single asic unit tests for show commands

[neethajohn]

- What I did
Added single asic unit tests for pfcwd show commands

- How to verify it

platform linux2 -- Python 2.7.16, pytest-3.10.1, py-1.7.0, pluggy-0.8.0 -- /usr/bin/python2
cachedir: .pytest_cache
rootdir: /sonic/src/sonic-utilities/tests, inifile: pytest.ini
plugins: cov-2.6.0
collected 8 items                                                                                                                                                                                                                       

tests/pfcwd_test.py::TestPfcwd::test_pfcwd_show_config PASSED                                                                                                                                                                     [ 12%]
tests/pfcwd_test.py::TestPfcwd::test_pfcwd_show_config_single_port PASSED                                                                                                                                                         [ 25%]
tests/pfcwd_test.py::TestPfcwd::test_pfcwd_show_config_multi_port PASSED                                                                                                                                                          [ 37%]
tests/pfcwd_test.py::TestPfcwd::test_pfcwd_show_config_invalid_port PASSED                                                                                                                                                        [ 50%]
tests/pfcwd_test.py::TestPfcwd::test_pfcwd_show_stats PASSED                                                                                                                                                                      [ 62%]
tests/pfcwd_test.py::TestPfcwd::test_pfcwd_show_stats_single_queue PASSED                                                                                                                                                         [ 75%]
tests/pfcwd_test.py::TestPfcwd::test_pfcwd_show_stats_multi_queue PASSED                                                                                                                                                          [ 87%]
tests/pfcwd_test.py::TestPfcwd::test_pfcwd_show_stats_invalid_queue PASSED                                                                                                                                                        [100%]

Config commands pending on the multi asic db mock

[jleveque]

One minor comment.

Very nice resuse of code, BTW. Very clean! 👍

[smaheshm]

Looks good! Waiting for unit test result.

[smaheshm]

retest this please

[abdosi]

@neethajohn Please create PR for 201911

[smaheshm]

@neethajohn @abdosi Will create one PR for both #1080 and #1085

[smaheshm]

@neethajohn Please create PR for 201911

#1102