Allow to dedicate a journal by OSD.

Please note that now you have to specify the full path of the device, ie /dev/something (for OSD and for journal).
@AlexandreNo, @EmilienM is right about having to manage backward compatibility. To fix the backward compatibility here, simply move `$osd_ceph = prefix($devices,'/dev/')` inside the `if is_array` and parametrize the `/dev/` part, leaving '/dev/' as the default value.

This way one will be able to run the module with its older version of the configuration file, and newer deployment will be done with absolute_path and the parameter set to '' (empty)