Adds new CI job definitions for CentOS 8:

- kolla-ansible-centos8-source
- kolla-ansible-centos8-binary
- kolla-ansible-centos8-source-ceph-ansible
- kolla-ansible-centos8-source-cinder-lvm
- kolla-ansible-centos8-source-mariadb
- kolla-ansible-centos8-source-bifrost
- kolla-ansible-centos8-source-zun
- kolla-ansible-centos8-source-swift
- kolla-ansible-centos8-source-scenario-nfv
- kolla-ansible-centos8-source-ironic
- kolla-ansible-centos8-binary-ironic
- kolla-ansible-centos8-source-masakari
- kolla-ansible-centos8-source-cells

The following jobs are added to the check pipeline:

- kolla-ansible-centos8-source
- kolla-ansible-centos8-binary
- kolla-ansible-centos8-source-cinder-lvm
- kolla-ansible-centos8-source-mariadb
- kolla-ansible-centos8-source-zun
- kolla-ansible-centos8-source-swift
- kolla-ansible-centos8-source-scenario-nfv
- kolla-ansible-centos8-source-ironic
- kolla-ansible-centos8-binary-ironic
- kolla-ansible-centos8-source-cells

The following jobs are not yet passing so are not added to the check
pipeline:

- kolla-ansible-centos8-source-ceph-ansible
- kolla-ansible-centos8-source-bifrost
- kolla-ansible-centos8-source-masakari

The kolla-ansible-centos8-source job is added to the gate.

Upgrade jobs will be added when CentOS 8 support exists in Train.

Depends-On: https://review.opendev.org/704337
Depends-On: https://review.opendev.org/704848
Depends-On: https://review.opendev.org/704965



Co-Authored-By: Mark Goddard <mark@stackhpc.com>

Change-Id: Ibd806feee71721b122b77d7eff33228ca1cc2853
Partially-Implements: blueprint centos-rhel-8