ustream ci: Use infra scripts to build testing images

As we do not use molecule features, using a Dockerfile and the
ansible-freeipa deployment roles is enough to create the container
testing images.

This patch removes the usage of molecule in favor of the custom
ansible-freeipa image building script, which allow us to have a similar
process for creating images both on the ustream CI, or on a developer's
environment.

Also, CentOS 7 is removed from the build script, as it in not possible
to run CentOS 7 containers with current versions of systemd.