Added a note that the ceph-osd command requires the cluster option.

Signed-off-by: Christopher O'Connell <jwriteclub@gmail.com>
Merge pull request #1293 from jwriteclub/doc-update-ceph-osd-note

Added a note that the ceph-osd command requires the cluster option.

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>