While supporting both CentOS 7 and 8, we used the tag 'master-centos8'
for CentOS 8 images. We are now ready to drop CentOS 7 support, and
Kolla is switching to publish CentOS 8 images using the master tag on
the master branch, so we should use this.

Depends-On: https://review.opendev.org/713265

Partially-Implements: blueprint centos-rhel-8

Change-Id: I07d2c285e3214a6dc827a8e8eacf263048ee099b