L2 Model additions to support DVR

This patch introduces the models, the DB migrations
and the config options required by the L2 layer to
support DVR east/west traffic.

These changes will be used by the control-plane made
of ML2, L2pop and L2 agent.

Two new configuration options have been introduced:
'dvr_base_mac' is used to set DVR MAC addresses apart
from tenant ones (every distributed router will have
ports being created on compute hosts) and
'enable_distributed_routing' is used to enable dvr
support in the L2 agent. This gives the capability of
rolling out the dvr functionality in stages.

Partially-implements: blueprint neutron-ovs-dvr

DocImpact

Change-Id: Iab6505f239d2c4c9bcbf4e32a292d7b4b5320c8e
Authored-by: Vivekanandan Narasimhan <vivekanandan.narasimhan@hp.com>
Co-Authored-By: Armando Migliaccio <armamig@gmail.com>