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
3 files changed