Add _get_router_ports_by_subnet_id
Add _get_router_ports_by_subnet_id

This patch add the function, _get_router_ports_by_subnet_id, to find the
router interfaces in the given port list with the subnet ID.

Signed-off-by: Taku Fukushima <48ddf7139da17a0278f1f27e611ac6b9304f94b2@midokura.com>

tfukushima/midonet-kubernetes,midonet/midonet-kubernetes-prototype,tfukushima/midonet-kubernetes,midonet/midonet-kubernetes-prototype