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
Cleanup lib/ironic

This moves around a bunch of functionality and attempts to isolate setup
steps into discrete functions (new or existing), making them easier to
consume from outside of Devstack (ie, Grenade).

Change-Id: I480167dcc008506ec2fe8c412db4114b74496e60
Merge "Cleanup lib/ironic"
Merge "Allow to use flat providernet for public network"