set default for --type to an empty list

The action is set to append but the default is still None so if --type
is not provided on the command line the option's value isn't iterable
and trying to use it causes an exception.

Change-Id: I17e027088db305fac51a557e7005293d904cf890
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

openstack_releases/cmds/list_deliverables.py

@@ -81,6 +81,7 @@ def main():
     )
     parser.add_argument(
         '--type',
+        default=[],
         action='append',
         choices=list(sorted(validate._VALID_TYPES)),
         help='deliverable type, such as "library" or "service"',