Introduce Stack Remove Members Operation, Interceptable by Parent Resource

Introduce into the Heat API an operation that is given a stack ID and a list of member resource IDs (you might think of these as resource names, but see http://docs.openstack.org/developer/heat/template_guide/hot_spec.html#resources-section), and removes those members from that stack --- giving the parent resource a chance to intercept the operation. That is, if the given stack SG is a nested stack ---- call the parent stack SP, and let RP be the ID of the resource backed by stack SG --- and RP has a handle_remove_members method then that method will be called to do the operation. Otherwise the operation will be implemented by: locking the stack, fetching the stack's current template, removing the identified members (if any) that exist in the template, doing a stack update with the revised template and current environment if anything was removed, and unlocking the stack.