===Set bridge_mappings for linux bridge===
The external network physnet needs a bridge_mapping to the public
bridge when the L2 agent is responsible for wiring.
===Add PUBLIC_PHYSICAL_NETWORK to flat_networks===
This network must be present in the ML2 flat_networks config if
flat_networks is specified.
===Set ext_gw_interface to PUBLIC_BRIDGE in provider net case===
ext_gw_interface must be a bridge in a bridge_mapping when
Q_USE_PROVIDERNET_FOR_PUBLIC is used.
Closes-Bug: #1605423
Change-Id: I95d63f8dfd21499c599d425678bf5327b599efcc
6a42a85b56