Adjust RBD delete log level

Set notice of volume no longer existing in RBD backend when attempting
to delete to log INFO level. Also move a comment next to related code.
This is a follow-up to suggestions from
3de7da12d1098ef777305099e5f4a039e536bf99.

Change-Id: I3ff854e0ead79e208106a371e14ec6ac7603d387
Merge "Adjust RBD delete log level"