use private ip if public dns name is not available

[jmoline]

I'm not sure how common a use case it is, but our instances do not get public ip or dns names, so this is just an automatic fallback to private ip if public dns is not available.

Edit: I've updated this, because I think it works better as a property that gets passed into the autoscale method, instead of just falling back to private ip when public dns isn't available (which might cause confusion).