Fix DynamicClusterTest.clusterLocationIsPassedOnToEntityStart

- failed in https://buildhive.cloudbees.com/job/brooklyncentral/job/brooklyn/919/consoleText
- caused by member of cluster having parent of app; so was being 
  stopped twice: once by app stopping all its children, and once by
  resizing cluster to zero. But second effector call to stop 
  was only getting there after managementContext had been stopped.