Comment 2 for bug 1566321

This bug is discovered by [1].
After some trace, I found out that we surprisely didn't support length: {get_param: [lengths, '1']}
this can lead to length: {get_param: [lengths, {get_param: index}]} in [1] raise error and didn't know what to do in this RG case.
This means that we now have a way to import "%index%" to a list index in get_param

[1]https://review.openstack.org/#/c/301623/2