[scanner] implement the --list flag

cdist/argparse.py

@@ -486,6 +486,10 @@ def get_parsers():
         '-m', '--mode', help='Which modes should run',
         action='append', default=[],
         choices=['scan', 'trigger'])
+    parser['scan'].add_argument(
+        '--list',
+        action='store_true',
+        help='List the known hosts and exit')
     parser['scan'].add_argument(
         '--config',
         action='store_true',