Vxlan ecmp tunnel with priority Endpoints.[202012]

[siqbal1986]

This is a partial Change. It enables the primary endpoint tag and adds the data structures to keep track of Primary and secondary endpoints in a VLAN tunnel. Additional changes will follow in the same PR.

What I did This commit enables the "Primary" member of the Vxlan ECMP tunnel data model. This commit does not amend the existing behavior of the system. If the primary endpoints list is provided, it must be the same as the "endpoint" list. Why I did it To add support for Primary /Backup endpoint routing behavior. How I verified it Added a test case and existing tests are passing. Details if related "Please don't merge this until this message is removed."