Update minigraph parser to support type MIRROR_DSCP

[bingwang-ms]

Why I did it

This PR is to update minigraph parser minigraph.py to support type MIRROR_DSCP.
A new section defined in minigraph.xml as below will create an ACL table with type MIRROR_DSCP.

  <AclInterfaces>
       <AclInterface>
          <AttachTo>ERSPAN_DSCP</AttachTo>
          <InAcl>Everflow_dscp</InAcl>
          <Type>Everflow_dscp</Type>
        </AclInterface>
  </AclInterfaces>

How I did it

Update minigraph.py to support parsing EVERFLOW_DSCP into config_db.

How to verify it

1. Verified by UT
2. Verified by updating minigraph.xml and runing config load_minigraph on device.

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

• 201811
• 201911
• 202006
• 202012
• 202106

Description for the changelog

Update minigraph parser to support type MIRROR_DSCP

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

[qiluo-msft]

This PR could not be cleanly cherry-pick to 202012. Please submit another PR.