mon: osd find / metadata --format plain fallback

ceph --format plain osd find 1 (and metadata) are not implemented and
must fallback to the default (json-pretty).

http://tracker.ceph.com/issues/9538 Fixes: #9538

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2533 from dachary/wip-9538-format-plain

mon: osd find / metadata --format plain fallback

Reviewed-by: John Spray <john.spray@redhat.com>