Improve log message when port losts its vlan tag

In some cases it may be useful to log new vlan tag which is found
on the port when it losts old vlan tag which should is expected to
be there.
So this patch adds such value to the log message.

TrivialFix

Change-Id: I231e624f460510decc6d2237040c8bef207e2e8e

neutron/plugins/ml2/drivers/openvswitch/agent/ovs_neutron_agent.py

@@ -1797,9 +1797,11 @@ class OVSNeutronAgent(l2population_rpc.L2populationRpcCallBackTunnelMixin,
                 ):
                     LOG.info(
                         "Port '%(port_name)s' has lost "
-                        "its vlan tag '%(vlan_tag)d'!",
+                        "its vlan tag '%(vlan_tag)d'! "
+                        "Current vlan tag on this port is '%(new_vlan_tag)d'.",
                         {'port_name': port.port_name,
-                         'vlan_tag': lvm.vlan}
+                         'vlan_tag': lvm.vlan,
+                         'new_vlan_tag': port_tags[port.port_name]}
                     )
                     changed_ports.add(port.vif_id)
         if changed_ports: