Add L3 Extension for Distributed Routers

This patch introduces the model and extension
framework for implementing distributed virtual
routing on top of Open vSwitch.

A new admin-only 'distributed' (as opposed to a
'centralized' or legacy-mode) attribute is added
to the API router resource. It is possible to convert
an existing (centralized) router to a distributed
one; the opposite conversion, even though allowed by
the API, may not be honored by the underlying
plugin implementation and an appropriate error will
be reported.

When creating a router (regardless of the user role),
Neutron will rely on a system wide configuration, whose
default currently allows to create 'centralized' routers.

Tests are added for basic unit coverage; when the first
building blocks for neutron-testing-refactor
are complete, functional testing will be added.
This is because we should be moving away from how
extension tests have been done up until now.

Partially-implements: blueprint neutron-ovs-dvr

DocImpact

Change-Id: I7227fbe2718eba6665a5afb5dcaaaa77b341091f
Authored-by:    Swaminathan Vasudevan <swaminathan.vasudevan@hp.com>
Co-Authored-By: Armando Migliaccio <armamig@gmail.com>
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:
one is used to set DVR MAC addresses apart from
tenant ones (every distributed router will have ports
being created on compute hosts) and the other 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
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
Merge DVR patches together