Don't allow set cronExecutionUser

ZEPPELIN-3045 also try to fix the issue, but seems no update for a long time. This is a critical security issue for zeppelin. The cron module may need to revisit to redesign and improve, but as a quick fix, I think we can just don't allow set cronExecutionUser, and just run the note via the note owner. \cc moon felixcheung prabhjyotsingh@apache.com jongyoul thoughts ?