Use the GITLAB_REF_SLUG instead of GITLAB_{CE,EE}_VERSION for {CE,EE}_ASSETS_TAG
This needs https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23435.
This allow some CE/EE images to be built with a different assets image than their own, which sometimes isn't built at all (e.g. QA branches).
Edited by Rémy Coutable