Implement handle_remove_members for OS::Heat::AutoScalingGroup

The implementation of the handle_remove_members method will scan the existing members to build the collection of members that must be removed. If there are some then, following a code path similar to that used by the scale up/down operations (but implementing the relevant semantics, of course), a revised template for the backing nested stack is generated and put into effect via the regular update operation on the nested stack.