[TripleO] Tear down of Train centOS 7 check jobs and integration line

Hello All, We have been running check/gate testing and integration lines for the Train release on both CentOS7 and CentOS 8. Following the work on CentOS 9 and the longevity of the Train release, we proposed removing the CentOS 7 Train jobs and building/testing changes to this release on CentOS 8 only. We floated this proposal with some interested parties and received no objections so we are beginning work on this tear down. We anticipate that removing the CentOS 7 jobs will allow patches to merge quicker and will free up resources for future work. Please respond if you have any questions or concerns. Thanks, Ronelle (for the TripleO CI team)