[sheepdog] Add confirm before format cluster

[Valerio Pachera]

Hi, I suggest to add a message before format the cluster (at least if
it contains any vdi).

---
collie vdi create test 1G
collie vdi list
  Name        Id    Size    Used  Shared    Creation time   VDI id  Copies  Tag
  test         1  1.0 GB  0.0 MB  0.0 MB 2012-12-12 17:00   7c2b25
2
collie cluster format
  using backend farm store
---

This just to avoid a mistake that can be done if working late and being tired:
instead of typing the harmless
  'collie cluster info'
type
  'collie cluster format'

...and loose everything!