This change modifies the CentOS build CI jobs to use a CentOS 8 host
image.
NOTE: This does not affect the version of CentOS used as the base image
in containers built by kolla.
Partially-Implements: blueprint centos-rhel-8
Partially-Implements: blueprint centos-rhel-python3
Change-Id: Ie25b54a4e6116ed2bcb9c3b5c44eb6edbd2e6b49