[cherry-pick][202012] Update db_migrator to support pfcwd_sw_enable

[bingwang-ms]

Signed-off-by: bingwang wang.bing@microsoft.com

What I did

This PR is to cherry-pick changes in PR #2087 to 202012 branch after resolving conflicts.
This PR is to update db_migrator to support pfcwd_sw_enable.
Currently, table pfc_enable is to specify on which queue to enable PFC and pfc watchdog. As we are going to add two extra lossless queues on which on watchdog is enable(HLD sonic-net/SONiC#950), a new table is required to specify on which queue to enable PFC watchdog. That is pfcwd_sw_enable.

Table	Description
pfc_enable	Specify on which queues to enable PFC
pfcwd_sw_enable	Specify on which queues to enable software PFC watchdog

HLD DSCP remapping

Change in orchagent Update orchagent to support new field pfcwd_sw_enable

How I did it

Update db_migrator.

How to verify it

1. Verified by UT

python3 setup.py test --addopts tests/db_migrator_test.py 

===================================================================================================================== 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 75 items                                                                                                                                                                                                                                             

tests/db_migrator_test.py ...........................................................................                                                                                                                                                    [100%]

================================================================================================================== 75 passed in 16.75 seconds ==================================================================================================================

3. Verified by copying the updated db_migrator.py to a SONiC box and run db_migrator.py -o migrate

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)