Using the cinderclient 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 `cinder --os-volume-api-version 3.40`.
Added a default value to `func` and an empty value to
`command` as placeholders so that a help message is
shown instead of the Python error.
Change-Id: Idb51e8635b97f0da2976f3268d5e19100ec77203
Closes-Bug: #1867061