Make make overcloud.yaml DTRT always.

Change-Id: I3aebf49c87434026b9eea9de9a67cb3196bc48b2
Support neutron.flat-networks in overclouds.

This is needed to allow configuring the template in
I9fa923b63033edb694720bfe5fc756a7c0fbfd2a.

Change-Id: I65810db156cb3d93291ac56fcf96e3ed2c87e1b2