Investigate the usage of /oldroot in the shutdown sequence

[aimxhaisse]

On a Ubuntu LTS image, when 'rebooting' the node, we have a warning about /oldroot still being used:

 * Stopping rsync daemon rsync                                           [ OK ]
 * Asking all remaining processes to terminate...                        [ OK ]
 * All processes ended within 1 seconds...                               [ OK ]
 * Deactivating swap...                                                  [ OK ]
mount: / is busy
 * Will now restart
umount: can't umount /oldroot: Device or resource busy
** Message: cmd check mode
** Message: [   49.251880] block nbd0: Receive control failed (result -32)
cmd check mode
[   49.332141] reboot: Restarting system