Use mro instead of hardcoding CUBA labels to retrieve the parents

I don't see a valid reason why we should use hardcoded labels instead of just calling 

`[x.cuba_key for x in inspect.getmro(Engine)[1:] if hasattr(x, "cuba_key")]`

More on this regard, most of the methods that we produce in the meta classes are identical copies of the exact same method that should be enough in the base class.
