Bug 1152171
This patch simply causes a 409 to be returned when attempting to
delete a subnet which has IPs on ports whose device_owner is
network:router_interface
This does not address a similar problem that arises when the ips for
a port owned by a router_interface are updated. This problem is hidden
when updating a port which has been assigned the gateway_ip by an
exception in the IP recycling process. However, it will hit when the
port was explicitly added to the router, since it will have an ip
different from the subnet's gateway ip.
Change-Id: I38dfd2639acd03db8d3f1f933495a456a4a8724f
e8dc595dde