Revert "remove external_network_bridge option"

This reverts commit faaf96bfb15c5f4c45a72b149dc6fe1e1f907a71.

Ironic jobs were still using this option, it needs to be
switched to an alternative first.

Change-Id: I1683d7cfa81f5fe2497cc7045e87f8b20fed4968
diff --git a/lib/neutron_plugins/ovs_base b/lib/neutron_plugins/ovs_base
index 558a7d5..523024e 100644
--- a/lib/neutron_plugins/ovs_base
+++ b/lib/neutron_plugins/ovs_base
@@ -96,6 +96,10 @@
 }
 
 function _neutron_ovs_base_configure_l3_agent {
+    if [ "$Q_USE_PROVIDERNET_FOR_PUBLIC" != "True" ]; then
+        iniset $Q_L3_CONF_FILE DEFAULT external_network_bridge $PUBLIC_BRIDGE
+    fi
+
     neutron-ovs-cleanup --config-file $NEUTRON_CONF
     if [[ "$Q_USE_PUBLIC_VETH" = "True" ]]; then
         ip link show $Q_PUBLIC_VETH_INT > /dev/null 2>&1 ||