[Tests] Enhance ZenUnicastDiscoveryTest

This started out as a simple correction to a missing setting problem, but go bigger into more general work on the ZenUnicastDiscoveryTets suite. It now works with both network and local mode. I also merge the different ZenUnicast test suites into a single place.

Closes #6835
Cleanup of the transport request/response messages

Now both TransportRequest and TransportResponse inherit from a base TransportMessage that holds the message headers and also now added the remote transport address (where this message came from).
Print trace log if not enough master nodes could be found.
[TEST] Ensure that one node is part of the cluster.
Docs: Fixed invalid word count in geodistance agg doc

Closes #6838