Add rollback for Cisco plugin update_port failure

Add rollback for Cisco plugin update_port failure

Fixes bug 1174433

This fix adds a rollback of the OVS plugin update_port operation
following a failure to update the port in the Cisco Nexus sub-plugin.

Also, this fix removes an unneccessary call to the Nexus sub-plugin's
update_network method within the Cisco plugin model layer's
update_network method (as well as the associated argument setup).
The Nexus sub-plugin's update_network is a no-op, so there's no
need to call it.

Change-Id: I10cba0d7f1d632c7f77eb6c4a94ea82b958bc6a2