Make DvrServerRpcCallback a separate callback class

RPC has a version of itself. In Neutron a plugin implements
several RPC interface, so a single RPC version doesn't work.
In Mixin callback class approach, RPC versioning depends on
each plugin implementation and it makes harder to maintain
RPC version appropriately. This patch series replaces mixin
RPC callback of server side with a separate class.

This commit handles server-side callback of DVR ML2 RPC interface.

Partial-Bug: #1359416
Change-Id: I1b6383f7b0af5d9aed18eda3a15f21d3504d0347
Merge "Make DvrServerRpcCallback a separate callback class"