ISPN-4760 Cluster registry doesn't work when rebalancing is disabled

https://issues.jboss.org/browse/ISPN-4760

* Rebalancing should always be enabled for the cluster registry cache
* Fix StateTransferSuppressIT
The RpcManager MBean was returning "N/A" when the cache manager wasn't clustered (which isn't actually possible, because the RpcManager MBean wouldn't exist), "null" when the cache topology didn't have a pending CH (i.e. no rebalance in progress), and it threw an exception when the cache didn't have a topology yet (although TBH I'm not sure the MBean that's possible either). I figured I'd simplify things a bit and return "N/A" every time we don't have a pending CH in the ISPN-4574 fix, but I forgot to update the test. 

Now that you've asked, I'm thinking I should go back to returning "null" when there is no rebalance in progress.