This is useful for validating deployments with OVN where it is not
required to have external networking attached to every chassis.
Any chassis that does not have external networking directly
attached will forward traffic destined for the external network
through a tunnel to a chassis that does.
It turns out we do need to set DNS on the overcloud subnet. It should be
the .2 of the CIDR under test and not the _admin_net's .2. There seems
there are security group rules in the way.
