Fix BigSwitch plugin to handle device_id re-use in porttracker code

Updates the BigSwitch port_tracker code to use the sqlalchemy merge method
instead of the add method for the port location tracking code to avoid primary
 key constraint errors if device-ids are moved or re-used

Fixes: bug #1208990
Change-Id: I06065ac888873b00f445d4356b17f084ee38c1ea
Merge "Fix BigSwitch plugin to handle device_id re-use in porttracker code"