I'm slightly uncomfortable with the KafkaCluster implementing Group, and delegating everything to `getCluster()`.
It will confuse things like `for (Entity m : getMembers()) m,getGroups();`: the member doesn't think it's in the group, but the KakfaCluster says it is.
I presume it's been done so that the KafkaCluster can be passed to things like a load balancer (or something like that).

(note delegating resize makes perfect sense).