Vxlan / L2population support to Linuxbridge Agent

This patch introduces VXLAN support for Linuxbridge agent alongside
with ml2 plugin support in linubridge mechnism driver.

A new vxlan configuration section is added for vxlan related parameters.

The agent also implements l2population RPC callbacks which allows ml2
plugin using l2population mechnism driver to populate vxlan forwarding
and neighbor tables following portbinding events. It allows agent to
respond locally to ARP requests for remote VMs and avoid dataplane based
learning. This should help limiting the use of multicast or flooding
for broadcast emulation in vxlan networks.

These changes should anyway have a limited risk, as agent behaviour
shouldn't be affected, except when vxlan is enabled alongside ml2 plugin.

Implements: blueprint l2-population

Change-Id: I99a961c53f9e451409f1affb079042936d8ae5c6
Merge "Vxlan / L2population support to Linuxbridge Agent"