prevent invalid deletion of ports using by L3 devices

prevent invalid deletion of ports using by L3 devices

bug 1039947

The bug noticed that an admin user could delete a port that stored
the underlying IP allocation for a floating IP.  This patch prevents
the direction deletion of ports via the API for ports that are used as
router interfaces, router gateways, of for floating IPs.

Add a unit test to check such an invalid delete, and also updates
unit tests to avoid them tripping over the new checks.

Change-Id: Ief28e3181583428d55259275a7c21151a4a4fa9b