Updated BBR to use peer group name as prefix.

[abdosi]

Why/What I did:

Currently BBR Manager looks for peer-group name to start with value define in constants.yml (https://github.com/Azure/sonic-buildimage/blob/master/files/image_config/constants/constants.yml#L40). Peer-name need to be exact string match.

The change in this PR is to make BBR configured for peer-group if it's name starts with (prefixed) with the string define in constants.yml.

This is needed because Peer name can have deployment id at the end with the string "DEPLOYMENT_ID_NUM".
Eg: TIER0_V4_DEPLOYMENT_ID_0

How I verify:

• Added Unit test cases (test___set_prepare_config_enabled_multiple_peers and test___set_prepare_config_disabled_multiple_peers)

adosi@68cd7f721b7f:/sonic/src/sonic-bgpcfgd$ pytest tests/test_bbr.py::test___set_prepare_config_enabled_multiple_peers
============================================================================================== test session starts ==============================================================================================
platform linux2 -- Python 2.7.13, pytest-3.0.6, py-1.4.32, pluggy-0.4.0
rootdir: /sonic/src/sonic-bgpcfgd, inifile: pytest.ini
plugins: cov-2.4.0
collected 29 items

tests/test_bbr.py .

---------- coverage: platform linux2, python 2.7.13-final-0 ----------
Name                             Stmts   Miss  Cover
----------------------------------------------------
bgpcfgd/__init__.py                  0      0   100%
bgpcfgd/__main__.py                  3      3     0%
bgpcfgd/config.py                   69     69     0%
bgpcfgd/directory.py                63     36    43%
bgpcfgd/frr.py                      49     49     0%
bgpcfgd/log.py                      15      6    60%
bgpcfgd/main.py                     53     53     0%
bgpcfgd/manager.py                  41     23    44%
bgpcfgd/managers_allow_list.py     425    425     0%
bgpcfgd/managers_bbr.py             95     59    38%
bgpcfgd/managers_bgp.py            192    192     0%
bgpcfgd/managers_db.py               9      9     0%
bgpcfgd/managers_intf.py            33     33     0%
bgpcfgd/managers_setsrc.py          44     44     0%
bgpcfgd/runner.py                   44     44     0%
bgpcfgd/template.py                 64     42    34%
bgpcfgd/utils.py                    19     19     0%
bgpcfgd/vars.py                      1      0   100%
----------------------------------------------------
TOTAL                             1219   1106     9%


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