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
Merge "Add l2 population base classes"