Heat should be able to cancel a remote Stack resource

[Zane Bitter]

In nested stack resources (i.e. subclasses of StackResource) we have implemented the handle_create_cancel() and handle_update_cancel() methods to allow operations on the nested stack to be cancelled in response to events occurring in the parent stack.

We should do the same for the OS::Heat::Stack (RemoteStack) resource, which uses the Heat API to manage a stack (sometimes in another region).

Previously the RPC API offered more cancellation options than the REST API, so that this couldn't be implemented in RemoteStack. That is no longer the case - the REST API now allows us to cancel without rolling back so we can duplicate the same functionality.