Use assertIn to check for subnet membership

For dual stacked networks, the gateway port will have multiple subnets
associated

Change-Id: I450f896f29051921b011e1643f1a1e24ed82ac08
diff --git a/tempest/api/network/test_routers.py b/tempest/api/network/test_routers.py
index a48271e..34650c5 100644
--- a/tempest/api/network/test_routers.py
+++ b/tempest/api/network/test_routers.py
@@ -194,7 +194,8 @@
         resp, public_net_body = self.admin_client.show_network(
             CONF.network.public_network_id)
         public_subnet_id = public_net_body['network']['subnets'][0]
-        self.assertEqual(fixed_ips[0]['subnet_id'], public_subnet_id)
+        self.assertIn(public_subnet_id,
+                      map(lambda x: x['subnet_id'], fixed_ips))
 
     @test.attr(type='smoke')
     def test_update_router_set_gateway(self):