Define PUBLIC_BRIDGE in the main Neutron lib

This way, it can be used by both OVS and Linux Bridge

Change-Id: Iea5a8bb720d327b69f64791a23d414d4cde2e3ea
Closes-Bug: #1460758
diff --git a/lib/neutron-legacy b/lib/neutron-legacy
index 18b0942..8879bd9 100644
--- a/lib/neutron-legacy
+++ b/lib/neutron-legacy
@@ -168,6 +168,10 @@
 ## Provider Network Information
 PROVIDER_SUBNET_NAME=${PROVIDER_SUBNET_NAME:-"provider_net"}
 
+# Define the public bridge that will transmit traffic from VMs to the
+# physical network - used by both the OVS and Linux Bridge drivers.
+PUBLIC_BRIDGE=${PUBLIC_BRIDGE:-br-ex}
+
 # Use flat providernet for public network
 #
 # If Q_USE_PROVIDERNET_FOR_PUBLIC=True, use a flat provider network