orchestration: tolerate NotFound in wait_for_stack_status

When waiting for a stack to transition to DELETE_COMPLETE status
it's possible to either get that status, or a NotFound exception
so catch the exception and return in that case, to avoid needing
try/except around the wait, or risking races in tests which wait
for DELETE_COMPLETE without a try/except (test_volumes.py)

Change-Id: I4ded172db3c8969075365a0dc6b60d8e3e7db71d
Partial-Bug: #1344989
Merge "orchestration: tolerate NotFound in wait_for_stack_status"