Add l2 population base classes

This patch initiates the blueprint l2-population
Implemented as a ml2 Mechanism driver.

OVS & LinuxBridge drivers will be added as dependencies.

Rebased on ML2 Portbinding.

Change-Id: Ia2345aa262ec791c9f38b6e41e1e4b46f69cadac
Vxlan / L2population support to Linuxbridge Agent

Vxlan support based on https://review.openstack.org/26516

This is a part of the blueprint l2-population

Change-Id: I99a961c53f9e451409f1affb079042936d8ae5c6