Update xml format to handle relative locations

New sample XML:
<vertical url_name="split_test_vert">
    <split_test url_name="split1" user_partition_id="0" group_id_to_child='{"0": "html/split_test_cond0", "1": "html/split_test_cond1"}'>
       <html url_name="split_test_cond0">condition 0</html>
       <html url_name="split_test_cond1">condition 1</html>
    </split_test>
</vertical>