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>
Modify L3 Agent for Distributed Routers

This patch is an enhancement to the existing L3 Agent.
This allows the L3 Agent to support distributed routers
by enhancing the router object to function across
multiple nodes.

Utilized two new types of namespaces:

- FIP to handle multiple VM fips and routers per node
- SNAT to handle centralized SNAT per router

Rules and tables are enhanced and added to support routing
across distributed routers without going to a centralized
router.

Finally, a new configuration param 'agent_mode' is introduced
and it controls what the L3 agent can do: the available values
are: 'legacy', 'dvr', 'dvr_snat' (more details inline).

The l3-scheduler uses the newly introduced agent_mode to
determine what L3 agent to select during the scheduling
process.

Partially-Implements: blueprint neutron-ovs-dvr

DocImpact

Change-Id: Icead821bb74372b15aac2e5cefe8ad7f08c037ab
Co-Authored-By: Rajeev Grover <rajeev.grover@hp.com>
L2 Agent-side additions to support DVR

This patch introduces changes to the L2 agent, whereby the L2 agent
relies on a DVR component that takes care of the port wiring and the
management of tunnels in face of topology changes due to the life
cycles or VM's as well as the life cycles of distributed virtual
routers.

Support for DVR needs to be explicitly enabled. Default behavior
remains unchanged.

Partially-implements: blueprint neutron-ovs-dvr

Change-Id: If75225898a6f0aeea8b0300b711ca7e01f6b4f9a
Add L3 Scheduler Changes for Distributed Routers

This patch implements the L3 Scheduler changes for the
Distributed Virtual Routers.

Partially-implements: blueprint neutron-ovs-dvr

Change-Id: I407c3d639ebdf885b1418bceac7cfc251e7eba1f
Co-Authored-By: Carl Baldwin <carl.baldwin@hp.com>
Co-Authored-By: Armando Migliaccio <armamig@gmail.com>
Merge DVR patches together