[tripleo] stable/train migration from CentOS-7 to CentOS-8

Greetings, I wanted to give everyone a heads up that the migration of upstream stable/train jobs from CentOS-7 to CentOS-8 [1] is close. CentOS-7 stable/train will continue to be tested in our 3rd party periodic pipeline and builds of CentOS-7 stable/train will be qualified here [2] Please let me know if you have any questions. Thanks! [1] https://review.opendev.org/#/c/738375 [2] https://review.rdoproject.org/zuul/builds?pipeline=openstack-periodic-integr...