Assert DELETE_COMPLETE on all stack delete cleanup

Currently stacks are deleted on integration test cleanup but no
polling is done for DELETE_COMPLETE, so the test will pass even though
the stack may be in a DELETE_FAILED state.

This change uses _stack_delete for all cleanup stack deleting, which
polls for complete.

Change-Id: Ic44b27130596cf3fa8616608a091b683ea131e0b
Closes-Bug: #1486303
diff --git a/functional/test_instance_group.py b/functional/test_instance_group.py
index a8494c2..b8bcc3d 100644
--- a/functional/test_instance_group.py
+++ b/functional/test_instance_group.py
@@ -225,7 +225,7 @@
             parameters={},
             environment=env
         )
-        self.addCleanup(self.client.stacks.delete, stack_name)
+        self.addCleanup(self._stack_delete, stack_name)
         stack = self.client.stacks.get(stack_name)
         stack_identifier = '%s/%s' % (stack_name, stack.id)
         self._wait_for_stack_status(stack_identifier, 'CREATE_FAILED')