BrooklynNode - unmanage on stop, fix abort logic

* Allow entities to be able to abort their stop effector. If pre-stop is aborted the machine will still be released.
* Unmanage BrooklynNode on stop

WARNING: Turns out that postStop wasn't wired up, it was never called. Nevertheless some entities (i.e. CouchDBNode, NginxController, JavaWebAppSoftwareProcess) override it which might introduce unwanted behaviour. Shall I remove the override in existing entities?

Why is this `Callable<String>`? If it only returns `null` then change to `Void` type.