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>

src/ceph-disk

@@ -2384,7 +2384,7 @@ def parse_args():
     zap_parser.add_argument(
         'dev',
         metavar='DEV',
-        nargs='*',
+        nargs='+',
         help='path to block device',
         )
     zap_parser.set_defaults(