updated "sample.ceph.conf":

I've made the following changes to simplify initial cluster configuration:

 * Removed deprecated option(s).
 * Removed overrides for default paths that are "not recommended to change".
 * Added comments with keys' descriptions, types and defaults.
 * Added references to relevant on-line docs;
   sorted keys according to their respective docs sections (pages).
 * Use different types of comments for annotations ('#')
   and for keys/values (';').
 * Added many useful keys.
 * Commented some lines to make sample config "neutral". The idea is to
   let users to un-comment what they need rather than remove sample sections
   that they surely won't be using as is.

Signed-off-by: Dmitry Smirnov <onlyjob@member.fsf.org>