Big Switch: Bind IVS ports in ML2 driver

Add support to bind IVS ports in the Big Switch ML2
mechanism driver. The backend controller will be checked
to determine if a host is connected using the Indigo vswitch.
If so, the mechanism driver will mark it as bound since
it will be provisioned by the backend controller.

Implements: blueprint bsn-ml2-bind-ivs
Change-Id: Ic481fc31c8c123899fddf8185c32f127dff53b7a
Merge "Big Switch: Bind IVS ports in ML2 driver"