plugin XEP-0060: raise an `NotFound` exception instead of StanzaError in `subscribe`: `exceptions.NotFound` is now raised in stead of `item-not-found` `StanzaError` for consistency with `getItems` and because it's easier to catch (condition has to be checked when `StanzaError` is received). rel 372

these certificates are used for demo purpose only. Those must be the same as in
backend_e2e (they are generated there)