ceph: flush stderr, stdout for sane output; add prefix

Aie.

e.g., ceph tell mon.* injectargs '--debug-ms 1'

 mon.a: injectargs:debug_ms=1/1
 mon.b: injectargs:debug_ms=1/1
 mon.c: injectargs:debug_ms=1/1

or

 osd.0: debug_ms=1/1
 osd.1: debug_ms=1/1
 osd.2: Problem getting command descriptions from ('osd', '2'), ENXIO
 osd.3: Problem getting command descriptions from ('osd', '3'), ENXIO
 osd.4: Problem getting command descriptions from ('osd', '4'), ENXIO
 osd.5: Problem getting command descriptions from ('osd', '5'), ENXIO

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