Add parameter for custom builder image
This MR:
- disables build of MariaDB and RabbitMQ, because we don't use it
- adds support for building containers with custom docker image (because of Python 3 dependencies)
- adds support for building Centos 7 and Centos 8 images at the same time
- adds job for building Train release in Centos 8 images (although not working until this commit https://github.com/openstack/kolla/commit/2def0be9a3c75e11a6cf99431bf917c54a11d9f3 is part of Train release - currently only in stable/train branch)
Edited by Tomáš Sapák