[202012] [cherry-pick] Apply DSCP_TO_TC_MAP from PORT_QOS_MAP|global to switch level

[bingwang-ms]

What I did
This PR is to cherry-pick #2314 to 202012 branch after resolving conflicts.

This PR is to update the code for applying switch level DSCP_TO_TC_MAP.
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": "[DSCP_TO_TC_MAP|AZURE]"
        }
}

The entry will be consumed by qosorch, and the specified map will be applied to switch.

Why I did it
This change is to ensure the correct DSCP_TO_TC_MAP is applied to switch level.

How I verified it
Verified by a new test case test_dscp_to_tc_map_applied_to_switch

collected 2 items                                                                                                                                                                                     

test_qos_map.py::TestDscpToTcMap::test_dscp_to_tc_map_applied_to_switch PASSED                                                                                                                  [ 50%]
test_qos_map.py::test_nonflaky_dummy PASSED                                                                                                                                                     [100%]

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