Fix binding:host_id is set to None when port update

when updating a port 'binding:host_id' is reset if not specified among
the parameter to be updated. As a result, a None value for
'binding:host_id' is sent from the notifier which might potentially
cause consumers to not work properly.

Closes-Bug: #1245310
Change-Id: Icfb5179940cca9f8a705eb36bdbfcbc8a421a272
Merge "Fix binding:host_id is set to None when port update"