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).
By doing this patch, you are breaking the backward compatibility with previous versions, because you delete the prefix. So if a deployer is not aware about this change, the Ceph configuration will be with "sda, sdb, sdc", etc. because he would have miss this change.
When you change something in Puppet, you have to take care about previous behaviors and manage them.