[sheepdog-users] Force cluster to start with a missing node

[Valerio Pachera]

I confirm that flushing the cache before doesn't change the issues:
- node 2 doesn't show the vdi (node 1 and node 3 they do).
- lot's of errors in the 'vdi check'
  (vdi is still *not* displayed on node 2)

Notes:
- to insert node 4 back into the cluster works fine
- all nodes are showing the right 'vdi node list'
- this time, I formated the cluster without 'quorum' option
- a second 'vdi check' gives no errors.