Use private DNS name if public is not available.

In VPC based deploymnets, there is not neccessarily a public DNS name for the monitored instance as it might live inside the VPC. In this scenario, the current implementation fails to construct a proper hostname.

However, the discovery might still be reachable via the private DNS name if the Turbine instance resides within the VPC as well. So, I added a fallback to the private DNS name.