conpaas-xtreemfs: allow to restart xtreemfs services

This patch modifies a few parts of XtreemFS manager's code to make it possible
to restart OSD/DIR/MRC services.

- Do not create a new volume if an existing one is already associated to a
  given OSD node.
- Specify that volumes do not need to be initialized again when restoring from
  a snapshot.
- Return a more compact data structure in get_service_snapshot.
- Bugfix in _start_osd: pass osd_uuid so that XtreemFS services can be scaled
  up again.


git-svn-id: svn://svn.forge.objectweb.org/svnroot/contrail/trunk/conpaas@5290 ef35cc4a-2640-4bbf-98bc-65920db7d8ae