Enable single host mode

This commit introduces a new config option 'osd crush chooseleaf type'.
With the help of this option and by setting it to '0' we tell Ceph to
store all the replicas on a single host. Basically we tell CRUSH to
iterate over disk and not over host.

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>
Merge pull request #81 from leseb/enh-testing-mode

Enable single host mode