Allow to use flat providernet for public network

The support of ancillary bridges (aka br-ex) is planned to be
dropped for ofagent.  This commit prepares the deprecation by
providing devstack support for an alternative way to connect
public network.

Related to blueprint ofagent-port-monitor
Change-Id: I6c2ce646070db0ed248665216071499a9b5567ab
diff --git a/lib/neutron_plugins/ovs_base b/lib/neutron_plugins/ovs_base
index 616a236..8375bb6 100644
--- a/lib/neutron_plugins/ovs_base
+++ b/lib/neutron_plugins/ovs_base
@@ -71,7 +71,11 @@
 }
 
 function _neutron_ovs_base_configure_l3_agent {
-    iniset $Q_L3_CONF_FILE DEFAULT external_network_bridge $PUBLIC_BRIDGE
+    if [ "$Q_USE_PROVIDERNET_FOR_PUBLIC" = "True" ]; then
+        iniset $Q_L3_CONF_FILE DEFAULT external_network_bridge ""
+    else
+        iniset $Q_L3_CONF_FILE DEFAULT external_network_bridge $PUBLIC_BRIDGE
+    fi
 
     neutron-ovs-cleanup
     # --no-wait causes a race condition if $PUBLIC_BRIDGE is not up when ip addr flush is called