Fix VTEP interface specification in proto spec

[mvachhar]

Currently the proto specifies a VTEP interface type and then abuses the interface addresses field to mean the addresses that should be terminated by any VTEPs created on the gateway. This is broken.

The correct way to do this is interfaces should only contain a list of interfaces that need to be created.

Elsewhere, we can specific the VTEP IP and use that.

We should drop the VTEP type of interface and just use a special VtepConfig block that specifies the information that the gateway uses to create VTEP interfaces.

[mvachhar]

This is a must fix for R1 so that we don't have to have a compatibility mode during ephemeral update in a customer environment.