Ability to use a directory for OSD instead of a disk

This commit implements a fourth scenario where we can directely use a
directory instead of a block device for the OSDs. The purpose of this
scenario is more testing-oriented. Please note that we do not check
the filesystem underneath the directory so it is really up to you to
configure this properly. Declaring more than one directory on the
same filesystem will confuse Ceph.

Fixes: #14

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>