Fix undesirable raw Python error

Using the glanceclient without a subcommand while
passing an optional argument triggers the raw Python
error `ERROR: 'Namespace' object has no attribute
'func'`. This bug can be reproduced by issuing the
command `glance --os-image-api-version 2`.
Added a default value to `func` as placeholder
so that a help message is shown instead of the Python error.

Closes-Bug: #1903727
Change-Id: Ie4288262e408192310cbbc240bd1779b265a64fd

glanceclient/shell.py

@@ -166,6 +166,8 @@ class OpenStackImagesShell(object):
         parser.add_argument('--os_image_api_version',
                             help=argparse.SUPPRESS)
 
+        parser.set_defaults(func=self.do_help)
+
         if osprofiler_profiler:
             parser.add_argument('--profile',
                                 metavar='HMAC_KEY',