Retrieve fresh network DB data before getting it in ml2

The DB objects that have join relationship with network, will be
acquired once during a DB transaction.
However, if they are updated after the first acquisition, the network
DB model object maintains the older value, and doesnt update it.

After cd7d63bde92e47a4b7bd4212b2e6c45f08c03143, the updated network
is retrieved from the db causing this bug.
This patch retrieves the latest data for these DB objects so that
updated information can be used.

Change-Id: Ic8935008515bed687ad3abd00c8ccaf656a30452
Closes-Bug: #1649503
diff --git a/neutron/tests/tempest/api/test_qos.py b/neutron/tests/tempest/api/test_qos.py
index 4f03557..184c092 100644
--- a/neutron/tests/tempest/api/test_qos.py
+++ b/neutron/tests/tempest/api/test_qos.py
@@ -177,8 +177,8 @@
             self.assertIn(rule, actual_rule_types)
 
     def _disassociate_network(self, client, network_id):
-        client.update_network(network_id, qos_policy_id=None)
-        updated_network = self.admin_client.show_network(network_id)
+        updated_network = client.update_network(network_id,
+                                                qos_policy_id=None)
         self.assertIsNone(updated_network['network']['qos_policy_id'])
 
     @decorators.idempotent_id('65b9ef75-1911-406a-bbdb-ca1d68d528b0')