Update_port should expect MACAlreadyExists

Conductor manager's update_port() method should include
exception.MACAlreadyExists in the list of expected exceptions.  This
will stop logging it as a traceback on the Conductor when a client
attempts to update a port and change its address to one that is already
registered.

Change-Id: Ic575e03da566c7b44bf6b72ac171633d80165ffc
Closes-Bug: #1348676
Merge "Update_port should expect MACAlreadyExists"