RADIUS user.
        :type username: str | unicode
        :param policies: List of policies associated with the user. This parameter is transformed to a comma-delimited
            string before being passed to Vault.
        :type policies: list
        :param mount_point: The "path" the method/backend was mounted on.
        :type mount_point: str | unicode
        :return: The response of the register_user request.
        :rtype: requests.Response
        NzT"policies" argument must be an instance of list or None, "{policies_type}" provided.)