Add OVN_BRIDGE_MAPPINGS - support extra bridge

Add's the OVN_BRIDGE_MAPPINGS variable to ovn_agent.
Uses the same format as OVS_BRIDGE_MAPPINGS, it defaults
to "$PYSICAL_NETWORK:$PUBLIC_BRIDGE".

This enables use of providernet for public network and
setting up additional bridges, for example a for baremetal.

Example:
  Q_USE_PROVIDER_NETWORKING="True"
  OVS_PHYSICAL_BRIDGE="brbm"
  PHYSICAL_NETWORK="mynetwork"
  PUBLIC_PHYSICAL_NETWORK="public"
  PUBLIC_BRIDGE="br-ex"
  OVN_BRIDGE_MAPPINGS="public:br-ex,mynetwork:brbm"

Change-Id: I37317251bbe95d64de06d6232c2d472a98c0ee4d
diff --git a/lib/neutron_plugins/ovn_agent b/lib/neutron_plugins/ovn_agent
index 3526ccd..c51b708 100644
--- a/lib/neutron_plugins/ovn_agent
+++ b/lib/neutron_plugins/ovn_agent
@@ -288,7 +288,7 @@
 function create_public_bridge {
     # Create the public bridge that OVN will use
     sudo ovs-vsctl --may-exist add-br $PUBLIC_BRIDGE -- set bridge $PUBLIC_BRIDGE protocols=OpenFlow13,OpenFlow15
-    sudo ovs-vsctl set open . external-ids:ovn-bridge-mappings=$PHYSICAL_NETWORK:$PUBLIC_BRIDGE
+    sudo ovs-vsctl set open . external-ids:ovn-bridge-mappings=${OVN_BRIDGE_MAPPINGS}
     _configure_public_network_connectivity
 }