quantum-debug configuration fails on missing PUBLIC_BRIDGE

The Nicira plugin does not require an L3 agent, however the
PUBLIC_BRIDGE is needed by the quantum-debug tool as well as
nicira third_party configuration.

Fixes bug #1190062

Change-Id: Ie60f8f340e034a35a65e54518838c1d751a481a8
diff --git a/lib/quantum_plugins/nicira b/lib/quantum_plugins/nicira
index 7795eed..f055866 100644
--- a/lib/quantum_plugins/nicira
+++ b/lib/quantum_plugins/nicira
@@ -46,7 +46,7 @@
 }
 
 function quantum_plugin_configure_debug_command() {
-    :
+    sudo ovs-vsctl --no-wait -- --may-exist add-br $PUBLIC_BRIDGE
 }
 
 function quantum_plugin_configure_dhcp_agent() {