Export portinfo thru portbinding ext in NEC plugin

Export portinfo thru portbinding ext in NEC plugin

blueprint nec-port-binding

* Add host-id support in port-binding extension.
* Expose portinfo thourgh binding:profile attr in a port.
  portinfo is a mapping between neutron port id and OpenFlow switch
  physical information (datapath_id and port_no)

It changes the following in portinfo db model
* Add cascade on delete to delete an associated portinfo
  when deleting the port.
* Use joined query for portinfo model to retrieve an associated
  portinfo when querying a port.

Change-Id: Id88d93dc0770a1290714436324b1b53c0b023eeb