BrooklynNode entity safe STOP effectors

Effectors to stop BrooklynNode forcing the user to choose explicitly whether to stop the running apps or not. Refuse to STOP if there are existing children (possibly mirroring nodes added by another entity)
The test models the logic in testCanStartAndStop where stopping the app is confirmed with SERVICE_UP == false. Nevertheless I am adding a check with getDriver().isRunning() (let's see what you think of it).