Use service loader to find KafkaCluster implementations

[k-wall]

It occurs to me that the junit ext ought to be applying the service loader pattern to KafkaCluster with container and in-vm implementations contributed from separate modules. The common code that relies on kafka server classes should looked at with a view to eliminating the dependency (mostly config constants IIRC). That way, if a user wanted to exclusively use the container-based implementation, they could avoid the kafka-server dependencies completely.

Like I say, I don't see any pressing need to do this now, but wanted to share the idea.

Originally posted by @k-wall in #206 (comment)