Enables BigSwitch/Restproxy ML2 VLAN driver

Refactors Bigswitch/Restproxy plugin by separating into
reusable libraries that can be used by the plugin as well
as the ml2 driver to proxy calls to the backend controller.

Enables basic unit tests for the ML2 driver.

Removes deprecated separate unplug/plug operations on ports.

Implements: blueprint bigswitch-ml2-driver
Change-Id: I4e22ba7e20ec4f405b9fd34a1bf08a48544f317d
Merge "Enables BigSwitch/Restproxy ML2 VLAN driver"