[202012] [cherry-pick] Update db_migrator to support PORT_QOS_MAP|global

[bingwang-ms]

What I did

This PR is to cherry-pick #2205 into 202012 branch after resolving conflicts.

This PR is to update the db_migrator to support generation of PORT_QOS_MAP|global.

After PR sonic-net/sonic-buildimage#10565, there will be two DSCP_TO_TC_MAP

• DSCP_TO_TC_MAP|AZURE is the default map, which is used at port level and switch level
• DSCP_TO_TC_MAP|AZURE_TUNNEL is used to remap the priority of tunnel traffic in dualtor deployment

To address the issue, an entry PORT_QOS_MAP|global will be added into config_db

"PORT_QOS_MAP": {
        "global": {
            "dscp_to_tc_map": "AZURE"
        }
}

To handle the upgrade from older image, db_migrator is updated to generate the entry.
There is no way to check if PORT_QOS_MAP|global is supported, so the migration is ran on all platforms. There is check in sonic-swss to check the switch capability code, so it will not cause issue.

How I did it

Update db_migrator to add a common migration step.

How to verify it

Verified by vstest

collected 76 items                                                                                                                                                                                    

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

===================================================================================== 76 passed in 18.26 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)

[stephenxs]

Comment left on the original PR #2205 is also applicable here.

[bingwang-ms]

Comment left on the original PR #2205 is also applicable here.

Thanks. I address the issue by checking the ASIC type. The table is only generated on one platform now. Also a test scenario is added to cover that.