Override of ``treebeard`` method to enforce the same root.

def mk_dropdown_tree(cls, model, root_node, for_node=None):
        '''
        Override of ``treebeard`` method to enforce the same root.
        '''
        options = []
        # The difference is that we only generate the subtree for the current root.
        logger.debug("Using root node pk of %s" % root_node.pk)
        cls.add_subtree(for_node, root_node, options)
        return options[1:]