ceph.conf: update sample

This commit adds:

* client admin socket
* change some default
* re-indent

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>
Merge pull request #3140 from leseb/sample-ceph.conf

ceph.conf: update sample

Reviewed-by: Sage Weil <sage@redhat.com>