sample.ceph.conf: some updates

This commit adds:
- a section for radosgw
- fix the replica count value (default is 3 since firefly)
- more OSDs options for performance and CRUSH

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>
actually the format is "/var/run/ceph/ceph.radosgw.{instance-name}.fastcgi.sock" and since we named the client "client.radosgw.gateway"