Use new Network.are_host_same() equivalency check

Replace the existing "is localhost" checks with the new, more reliable method.

Also, update the slave-registration validation logic to use the rsa_keys as the unique identifier for a host, rather than the hostname. This PR should significantly improve "clusterrunner deploy" robustness.
Want to fix this docstring (":str" -> ":type") while you're in here? :)