Add partial specs support in ML2 for multiprovider extension

ML2 provider network partial specs let admins choose some
multiprovider network attributes and let neutron choose remaining
attributes. This change provides the implementation for
multiprovider networks.

In practice, for VLAN/GRE/VXLAN segments provider:segmentation_id
choice can be delegated to neutron, in such case neutron try to find
a segment in tenant network pools which respects provided segment
attributes. For VLAN segments provider:physical_network choice can
also be delegated.

DocImpact

Implements blueprint provider-network-partial-specs

Change-Id: I1cf1441a179ec527674276b71e9924841f8570b6
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
Merge "Add partial specs support in ML2 for multiprovider extension"