Support private DNS names if public one is not available

[ppikula]

I made this change to allow using it for hosts that don't have public DNS names/IP addresses. If an instance doesn't have a public name, It tries to use an internal one. Ofc it is going work only when a proxy (bastion host) is configured.

If we have an LB in front of our application there is no need to keep hosts from the autoscaling group in the public subnet.