ceph-disk: zap needs at least one device

If given no argument, ceph-disk zap should display the usage instead of
silently doing nothing. Silence can be confused with "I zapped all the
disks".

http://tracker.ceph.com/issues/6981 fixes #6981

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #945 from dachary/wip-6981

ceph-disk: zap needs at least one device

Reviewed-by: Sage Weil <sage@inktank.com>