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

releasenotes/notes/fix-undesirable-raw-python-error-66e3ddaca7b72ae2.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    `Bug #1903727 <https://bugs.launchpad.net/python-glanceclient/+bug/1903727>`_:
+    Fixed raw Python error message when using ``glance`` without
+    a subcommand while passing an optional argument, such as
+    ``--os-image-api-version``.