Bind field to model class.

def bind_model_cls(self, model_cls):
        """Bind field to model class."""
        if self.model_cls:
            raise errors.Error('"{0}" has been already bound to "{1}" and '
                               'could not be rebound to "{2}"'.format(
                                   self, self.model_cls, model_cls))
        self.model_cls = model_cls
        return self