orchestration api test_stacks wait for deletion

Wait for delete to complete after triggering it, so we avoid
a conflict exception when the test _clear_stacks cleanup tries
to delete it again.

Change-Id: I64065ad8444d814e2cd7f162b869a2f8eedb1fad
Closes-Bug: #1344989
Merge "orchestration api test_stacks wait for deletion"