[CLIENT] Add internal liveness action

This commit adds a very lightweight action to the transport
serivce that allows to fetch clustername and the discovery node
from a node. This is used by transport clients to test liveness of
a node without using the nodesinfo API which can be blocking if management
threadpools are busy.
If I understand correctly, this is there for the case that the cluster services is not yet started and we get a liveness request. I wonder if it isn't cleaner to throw an exception in the handler (ElasticsearchIllegalStateException or similar) and not have to deal with the null disconode.