Configure NAT with baremetal when necessary

Configure NAT with baremetal when necessary

We currently only enable NAT on undercloud for virtual deployments.
However, there could be a case where a baremetal deployment also needs
NAT as it is not using an interface on the overcloud nodes with external
access. Therefore this patch changes the behavior to configure NAT when
the gateway of either the external or admin (when external is disabled)
network matches an IP assigned to the undercloud.

JIRA: APEX-605

Change-Id: I9c79af371913e6e5f0d39b433f68205bc7e106c5
Signed-off-by: Tim Rozet <trozet@redhat.com>