ceph.in: check ceph-conf returncode

ceph.in: check ceph-conf returncode

When running the ceph-conf command, we only check the stderr output. When
the ceph configuration file has duplicate values then the ceph daemon
command will fail because the stderr will contain the warning message but
the ceph-conf return code is 0.

$ ceph-conf --name mon.cephaio-1 --show-config-value admin_socket
warning: line 13: 'osd_memory_target' in section 'osd' redefined
/var/run/ceph/ceph-mon.cephaio-1.asok
$ echo $?
0

Fixes: https://tracker.ceph.com/issues/37664
Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>
(cherry picked from commit e69aea66ea5ba941e25eb42508e43e4670820cbd)