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