Comment 4 for bug 1087243

Devstack works fine just because the external network 'nova' is set as the router's gateway in admin context.
Outside of the admin context, the _check_for_dup_router_subnet routine will fail to find the subnet on the external network.

Actually there is no need at all to (re)load the subnet in that routine, so the easiest fix is probably just avoiding the load operations; this will also save a db access, which is always good.