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
16ac21f0da