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.
Merge pull request #902 from aledsage/fix/DynamicClusterTest

Review OK. Merging.