ceph: propagate do_command()'s return value to user space

We were returning '1' regardless of what do_command() returned in case
of error.  This would make building tools relying on command error codes
short of useless, and forced them to rely instead on error messages.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>