Remove running of one Heat test in system test 'Heat HA'

Heat OSTF 'test_autoscaling' is skipped in the HA mode, but in system tests
skipped test equal failed test, it is not correct in current situation. So need to don't
run test 'test_autoscaling' in the HA Heat system test.

Closes-Bug: #1371025

Change-Id: I5ab0495f4b57249ddce5de99512362f40d22aa2f