Run TripleO jobs on CentOS8 instead of CentOS7
Run TripleO jobs on CentOS8 instead of CentOS7

As we are cleaning up the c7 jobs and obselete featuresets [1].
This change replaces usage of CentOS7 in TripleO jobs by CentOS8.

[1]: https://review.opendev.org/q/topic:%22cleanup_featuresets%22+(status:open%20OR%20status:merged)

Change-Id: Id5bc15a22921a34399e926736d0a9a478598aff7

openstack/networking-bgpvpn,openstack/networking-bgpvpn,openstack/networking-bgpvpn