sample.ceph.conf update:

 * corrected URLs.
 * added [client] section.
 * more options and descriptions.
 * filestore settings were moved under [osd].
 * cephx settings to reflect lack of support for authentication in RBD client.
 * many minor corrections and updates.

Signed-off-by: Dmitry Smirnov <onlyjob@member.fsf.org>
Merge pull request #1748 from onlyjob/docs

sample.ceph.conf update:

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>