ML2 additions to support DVR

This patch introduces the changes necessary to
support DVR at Layer 2 with ML2, and L2pop.

Partially-implements: blueprint neutron-ovs-dvr

Change-Id: I709d877e7df83728868e68e023e6a37bb2e11704
Authored-by: Vivekanandan Narasimhan <vivekanandan.narasimhan@hp.com>
Merge "ML2 additions to support DVR"