ceph: old daemons output to outs and outbuf, combine

ceph: old daemons output to outs and outbuf, combine

When talking to old daemons, if a command succeeds, there may be
output on outs, outbuf, or both; combine them if there's no error,
and clear outs so it's not treated as stderr fodder.

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>