Small fixes for 'test_autoscaling' Heat test

- Skipped test in HA mode due to native Heat autoscaling
doesn't work in HA
- Increased timeout due to periodic Time Limit Errors in the
system tests

Closes-Bug: #1365431

Change-Id: I42161ee189550e39172c30bbd02402b8bcba7ad0
Merge "Small fixes for 'test_autoscaling' Heat test"