BSN: Bind external ports in ML2 driver

BSN: Bind external ports in ML2 driver

Add a binding capability to the Big Switch ML2 mechanism
driver to mark external ports as bound. Ports with the
owner type 'neutron:external_port' will be assigned to the first
VLAN segment available in the network. The port update operation
will then carry the HOST ID with the identifier to the backend
for the necessary fabric configuration.

Implements: blueprint bsn-ml2-ext-attach
Change-Id: I7b749ba5ef9a47b45be24f77656a10ed38e5e6ae