Add/Deletes group model instances to match the
        the given list of group names.

def create_or_update_groups(self):
        """Add/Deletes group model instances to match the
        the given list of group names.
        """

        for name in self.group_names:
            try:
                Group.objects.get(name=name)
            except ObjectDoesNotExist:
                Group.objects.create(name=name)
        Group.objects.exclude(name__in=self.group_names).delete()

        if self.verbose:
            names = [obj.name for obj in Group.objects.all().order_by("name")]
            sys.stdout.write(f"  Groups are: {', '.join(names)}\n")