[WFLY-2402] fix clustered attribute transformer

* get the hornetq-server resource by reading context.readResource(address)
  instead of PathAddress.EMPTY_ADDRESS.
* update the testClusteredTo120() test to use a hornetq-server resource
  with 1 cluster-connection (and thus clustered) to make sure the
  warning is logged as expected if the user wants to set the clustered
  attribute to false