[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.
did you run tests? I think this is going to make `ActionNamesTests` fail...I just added some docs to it to explain how we should categorize our transport actions and name them. Let me know if that is not clear ;)