config: make kolla work with oslo.config 7.0.0+

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)

kolla/common/config.py

@@ -223,7 +223,7 @@ _CLI_OPTS = [
                      ' push properly')),
     cfg.IntOpt('retries', short='r', default=3, min=0,
                help='The number of times to retry while building'),
-    cfg.MultiOpt('regex', types.String(), positional=True,
+    cfg.MultiOpt('regex', types.String(), positional=True, required=False,
                  help=('Build only images matching regex and its'
                        ' dependencies')),
     cfg.StrOpt('registry',