Fix Redmine/5748 - make defaults promises work in bundles using classes promise

The only qualitative difference between class_default and no_class default is that the former defines a class that it completely ignores.  This seems a lot of bother for something unlikely to make a difference.  Did you mean to condition the setting of a on this class, i.e.
```
vars:
  test::
    "a" string => "one";
```
or something like that ?