SubnetTier may cause dangling references in Brooklyn

[ygy]

It seems that SubnetTier may cause dangling references in Brooklyn when child entity doesn't have location.

When expunging SubnetTier its stop() method will be invoked. It will try to stop all children nodes. If child node is missing location then doStop() in org.apache.brooklyn.entity.software.base.lifecycle.MachineLifecycleEffectorTasks will miss to create a stopTask and subsequently locations will not be released.

Steps to reproduce:

1. Start a (Tomcat) deployment in vCD environment
2. Tomcat entity gets an empty location in the persistence layer, If vCD fails to provision VM.
3. Application and entity are set ON-FIRE (as it is currently in a customer environment)
4. Try to expunge the app via web UI
5. Brooklyn tries to expunge the app and its child nodes
6. Locations will not be removed since Tomcat entity has empty persisted location which prevents the release()