Adds exception handling around heat stack creation

Reasons:
- heat_client.stacks.create raises HTTPException which was not
  captured in heat work-flow.

Changes:
- Adds heat_exceptions.HTTPException in the exceptions captured
  while stack creation.

Change-Id: Ia87e1f6d08ac062e1085dd1c0b0c4197a74597d4
Closes-Bug: #1325049
Merge "Adds exception handling around heat stack creation"