Allow to use flat providernet for public network

The support of ancillary bridges (aka br-ex) is planned to be
dropped for ofagent.  This commit prepares the deprecation by
providing devstack support for an alternative way to connect
public network.

Related to blueprint ofagent-port-monitor
Change-Id: I6c2ce646070db0ed248665216071499a9b5567ab
Merge "Allow to use flat providernet for public network"