Do not delete subnets with IPs on router interfaces

Do not delete subnets with IPs on router interfaces

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