From oslo.config 7.0.0 release notes:
> Positional options are now required by default, to match argparse’s
> default behavior. To revert this behavior (and maintain optional
> positional arguments), you need to explicitly specify positional=True,
> required=False as part of the options definition.
So let's follow.
Change-Id: I77fdbcb905b1a7f2d535c3a37b2899118d1442c5
(cherry picked from commit aaa472bfa2)