Scaling policy support to CLUSTER_RESIZE

Scaling Policy does not support CLUSTER_RESIZE, but the Scenario S5 in https://docs.openstack.org/developer/senlin/developer/policies/deletion_v1.html says support.

This bug tracker is for errors with the documentation, use the following as a template and remove or add fields as you see fit. Convert [ ] into [x] to check boxes:

- [ ] This doc is inaccurate in this way: ______
- [ ] This is a doc addition request.
- [ ] I have a fix to the document that I can paste below including example: input and output.

If you have a troubleshooting or support issue, use the following resources:

 - Ask OpenStack: http://ask.openstack.org
 - The mailing list: http://lists.openstack.org
 - IRC: 'openstack' channel on Freenode

-----------------------------------
Release: 4.0.0.0b2.dev108 on None
SHA: 'Tue May 23 06:15:51 2017, commit f30e630
Source: https://git.openstack.org/cgit/openstack/senlin/tree/doc/source/developer/policies/deletion_v1.rst
URL: https://docs.openstack.org/developer/senlin/developer/policies/deletion_v1.html