Comment 4 for bug 1538166

Add to review:
https://review.openstack.org/#/c/274150/

I used option 3 ( metric instance) because cpu metric is not working scale_down policy - This metric counts the number of CPU cycles. It is ever increasing number.

In addition, instance metric reduces test up to 5 minutes.
if the Ceilometer team provides a more appropriate metric, I'll rewrite the test again with new metric.