Explicitly pass cluster name to "ceph-disk prepare"

Currently, it just works because we use the default cluster name, but if
it changes, then this will stop working.
Pass --zap-disk, not --zap to ceph-disk prepare

The real option is --zap-disk.
Merge pull request #68 from vuntz/ceph-disk-prepare

Make call to ceph-disk prepare more "correct"