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 b/lib/neutron
index e90ada8..ca9b788 100644
--- a/lib/neutron
+++ b/lib/neutron
@@ -303,6 +303,11 @@
     Q_USE_SECGROUP=False
 fi
 
+# OVN_BRIDGE_MAPPINGS - ovn-bridge-mappings
+# NOTE(hjensas): Initialize after sourcing neutron_plugins/services/l3
+# which initialize PUBLIC_BRIDGE.
+OVN_BRIDGE_MAPPINGS=${OVN_BRIDGE_MAPPINGS:-$PHYSICAL_NETWORK:$PUBLIC_BRIDGE}
+
 # Save trace setting
 _XTRACE_NEUTRON=$(set +o | grep xtrace)
 set +o xtrace