ML2 binding:profile port attribute

ML2 binding:profile port attribute

The ML2 plugin stores the binding:profile port attribute, defined as a
dictionary, in its ml2_port_bindings DB table. Since the plugin can
support a variety of MechanismDrivers with different needs for
binding:profile attribute content, the plugin will accept, store, and
return arbitrary key/value pairs within the attribute. As with the
binding:host_id attribute, updates to binding:profile trigger
rebinding.

Implements: blueprint ml2-binding-profile
Change-Id: I01cba8d09dde9de1c6160d0235b0d289eed91b29