[Bgpcfgd] Add unit tests

[shi-su]

- Why I did it
Add unit tests for bgpcfgd

- How I did it
Add unit tests for bgpcfgd and fix a minor bug in manager_intf.py found in testing

Test coverage with this PR

Name                             Stmts   Miss  Cover
----------------------------------------------------
bgpcfgd/__init__.py                  0      0   100%
bgpcfgd/__main__.py                  3      3     0%
bgpcfgd/config.py                   69      0   100%
bgpcfgd/directory.py                63      0   100%
bgpcfgd/frr.py                      49      1    98%
bgpcfgd/log.py                      15      2    87%
bgpcfgd/main.py                     53     53     0%
bgpcfgd/manager.py                  41     21    49%
bgpcfgd/managers_allow_list.py     425     29    93%
bgpcfgd/managers_bbr.py             84      0   100%
bgpcfgd/managers_bgp.py            192     33    83%
bgpcfgd/managers_db.py               9      0   100%
bgpcfgd/managers_intf.py            33      0   100%
bgpcfgd/managers_setsrc.py          44      7    84%
bgpcfgd/runner.py                   44     44     0%
bgpcfgd/template.py                 64      8    88%
bgpcfgd/utils.py                    19     14    26%
bgpcfgd/vars.py                      1      0   100%
----------------------------------------------------
TOTAL                             1208    215    82%

- How to verify it
Run unit tests locally and confirm all passed.

- Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[lguohan]

can you provide the code coverge data?

[shi-su]

can you provide the code coverge data?

Added coverage data in the PR description.