Add multi-journal support and fix collocation scenario

This commits brings the support of multiple journals where each journal
points to a specific OSD and vice-versa. The commit also clarifies the
usage of multi scenarios for both journal and osd_data.
In the meantime, it fixes the collocation scenario.

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>
Merge pull request #11 from leseb/wip-multi-journal

Add multi-journal support and fix collocation scenario