Set external ID on br-ex

This will enable Neurtron identify that the external bridge is a
Neutron bridge (this is required for bug 1192883)

Change-Id: I8ad1b0b3d93d5068beec2021abf9afbacf8c48ff
diff --git a/lib/neutron_plugins/ovs_base b/lib/neutron_plugins/ovs_base
index 0a53bff..2666d8e 100644
--- a/lib/neutron_plugins/ovs_base
+++ b/lib/neutron_plugins/ovs_base
@@ -67,6 +67,7 @@
 
     neutron-ovs-cleanup
     sudo ovs-vsctl --no-wait -- --may-exist add-br $PUBLIC_BRIDGE
+    sudo ovs-vsctl --no-wait br-set-external-id $PUBLIC_BRIDGE bridge-id $PUBLIC_BRIDGE
     # ensure no IP is configured on the public bridge
     sudo ip addr flush dev $PUBLIC_BRIDGE
 }