ML2: ODL driver sets port status

The OpenDaylight mechanism driver does not depend on an L2 agent to
plug the port. Now that nova waits for notification that the port
status is ACTIVE, the ML2 driver API is extended so that the mechanism
driver that binds a port can optionally set the port status, and the
OpenDaylight mechanism driver uses this to set the port status to
ACTIVE.

Closes-Bug: 1301449
Change-Id: I171c405f36b4f2354d9585e8ae3dfa50ddaa9a7e