As part of adoption of CentOS 8 in RDO, this patch is adding support to
test with it:
- Adapt run_tests.sh for CentOS8.
- Add the option to have different versions of puppet modules (needed
for puppet-mysql until we can update mariadb in CentOS7.
- Add centos8 playbooks and jobs definition.
Change-Id: Ie53a8cb5517651487436e24bf60f51bc531f16d6
yatinkarel
a344b9cd36