Add router check, subnet attached gateway IP update or deletion

Added a new test to ``RoutersTest``. If a subnet has a router
interface, the subnet gateway IP cannot be modified or deleted.
Both operations will raise a ``GatewayIpInUse`` exception.

Depends-On: https://review.opendev.org/c/openstack/neutron/+/904713

Related-Bug: #2036423
Change-Id: I46a39c53017589e23e03ceabc45c2f144ca2f3bb
2 files changed