Let OVSQuantumTunnelAgent sync with database

In daemon_loop, db.commit() is required to fetch
updated network status. Without this, ports are
stick to DEAD_VLAN_TAG.

Change-Id: I1a951d60eb75384676309e8db29ea7baa48055bc

quantum/plugins/openvswitch/agent/ovs_quantum_agent.py

@@ -640,6 +640,14 @@ class OVSQuantumTunnelAgent(object):
 
             old_vif_ports = new_vif_ports
             old_local_bindings = new_local_bindings
+            try:
+                db.commit()
+            except Exception as e:
+                LOG.info("Unable to commit to database! Exception: %s" % e)
+                db.rollback()
+                old_local_bindings = {}
+                old_vif_ports = {}
+
             time.sleep(self.polling_interval)