Set validation defaults to match undercloud default settings
In undercloud.conf, by default the dhcp ip address range is 20
(dhcp_start = 192.168.24.5, dhcp_end = 192.168.24.24).
This can be changed but is reasonable for POCs. The validation
default should match this, otherwise there will be validation
failures when run on existing deployments that haven't
changed the default.
(cherry picked from commit