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)
maybe replace lines 464 onwards with a simple `brooklynNode.invoke(eff, ...)`, return the task, and let the calling test inspect the tasks and children (adding a new `@Test testStopPlainThrowsException` to perform the existing test on lines 464-467) ?