Hi, a fresh clone of six-tools master branch followed by make results in the following error while attempting to yum install gaolang. On investigation it seems that the URL does indeed no longer exist.

http://vault.centos.org/centos/7/cloud/Source/openstack-queens/repodata/repomd.xml: [Errno 14] HTTP Error 404 - Not Found
Trying other mirror.
To address this issue please refer to the below knowledge base article 

https://access.redhat.com/articles/1320623

Peter Smith