Orchestration autoscaling improve status polling

This change includes the following improvements to the
autoscaling test:
- On stack create status timeout, override the error status so
  that FAILED stacks fail fast
- Improve the error message in assertScale
- Delete the stack at the end of the scenario, and confirm the delete
  succeeded

Change-Id: Ia220e3a2b422472ac7c6d29f9a5a61bd7c4987c3
Merge "Orchestration autoscaling improve status polling"