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>
Merge pull request #2339 from leseb/sample-ceph-conf-addition

sample.ceph.conf: some updates