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)
better now, `eff` is used :)   however we still don't assert whether the app was stopped or not -- would be nice to test that, but not essential.  (i've confirmed manually.)