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
1eb0bbbed7