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 cd7d63bde9, 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.

In order to pass functional tests, get_network is still called instead
of _make_network_dict as in original change.

Change-Id: Ic8935008515bed687ad3abd00c8ccaf656a30452
Closes-Bug: #1649503
(cherry-pick from commit e5ad765484)

neutron/plugins/ml2/plugin.py

@@ -809,6 +809,11 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
             self.type_manager.extend_network_dict_provider(context,
                                                            updated_network)
 
+            # ToDO(QoS): This would change once EngineFacade moves out
+            db_network = self._get_network(context, id)
+            # Expire the db_network in current transaction, so that the join
+            # relationship can be updated.
+            context.session.expire(db_network)
             updated_network = self.get_network(context, id)
 
             # TODO(QoS): Move out to the extension framework somehow.

neutron/tests/tempest/api/test_qos.py

@@ -176,8 +176,8 @@ class QosTestJSON(base.BaseAdminNetworkTest):
             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'])
 
     @test.idempotent_id('65b9ef75-1911-406a-bbdb-ca1d68d528b0')