Add device_owner to gateway port search criteria

When veryfing a gateway, search only for those ports
in the router which are tagged with proper device_owner
attribute.

The change addresses a case when networking-opencontrail
needs more ports with the same device_id and network_id
for any created router. This is needed for compatibility
with TungstenFabric.

Change-Id: If1c711547542460ebb3f3f9371a09669edb6ce5d
diff --git a/tempest/api/network/admin/test_routers.py b/tempest/api/network/admin/test_routers.py
index a7355f3..6ce86fb 100644
--- a/tempest/api/network/admin/test_routers.py
+++ b/tempest/api/network/admin/test_routers.py
@@ -111,7 +111,8 @@
     def _verify_gateway_port(self, router_id):
         list_body = self.admin_ports_client.list_ports(
             network_id=CONF.network.public_network_id,
-            device_id=router_id)
+            device_id=router_id,
+            device_owner="network:router_gateway")
         self.assertEqual(len(list_body['ports']), 1)
         gw_port = list_body['ports'][0]
         fixed_ips = gw_port['fixed_ips']