Add Netconf support for insert in ordered-by user lists

We need for a specific case in acls:

https://git.fd.io/hc2vpp/tree/acl/acl-api/src/main/yang/ietf-access-control-list@2016-07-08.yang

List ace under container access-list-entries is ordered-by user, which should allow us to insert acl rules at specific points in the list.