mon: 'osd create <uuid>'

mon: 'osd create <uuid>'

Make the osd create command idempotent by providing a uuid.  If you call it
multiple times with the same (or some other existing) uuid you'll get back
the osd id that is already using it.

Drop support for 'osd create <id>', which was mostly useless and
non-idempotent anyway.

Signed-off-by: Sage Weil <sage@newdream.net>