I don't think it hurts to repeat the recommendation to start the receptions here.

When you register the actors you use `ClusterReceptionistExtension(system).registerService`, which of course also load the extension if not already done, but if you have a node that doesn't have any actors that are registered the extension will not be loaded and things will not work.

I suggest that you add description of the akka.extensions conf property above, and change ClusterClientSpec to explicitly use `ClusterReceptionistExtension(system)` instead of the `def receptionist` (remove the `def receptionist`).