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)
and add `@ApiParam` ?  i now appreciate that the API here previously took longs so you just followed that pattern, but i think we should move towards consistency.  to preserve backwards compatibility we could have

     @ApiParam(name = "delay", value = "Minimum delay before system.exit", required = false)
     @FormParam("delay") @DefaultValue("250ms") String delay, 
     @ApiParam(name = "delayMillis", value = "Deprecated delay before timeout", required = false)
     @FormParam("delayMillis") Long delayMillis

and then in the code have an `if (delayMillis!=null) { /* log warn of deprecated, and use this value */ }`

?