Switch overcloud Ironic to OVN baremetal and distinguish undercloud
ironic config from overcloud ironic config. Currently undercloud is
using ML2/OVS and will need 'openvswitch' driver there while the
overcloud default driver has now been shifted to ML2/OVN.
Hence we need to split the ironic environment to cover both scenarios.
Change-Id: I754edb68a715b6386a77684d9a72af64e4f188b2