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)
oops sorry, for REST calls type-coercion won't apply.  but the pattern elsewhere is to use strings and then `Time.parseTimeString`, as in `EffectorResource.invoke`