Support QA image tagged with rails ref format used by auto-deploy

Right now, when a GitLab image with reference like dev.gitlab.org:5005/gitlab/omnibus-gitlab/gitlab-ee-qa:15.4-45fce49eaf2 is specified, the QA image tag is computed to be 15.4-45fce49eaf2. As part of &8661 (closed), we are changing the QA image tag to be just the rails ref without version component. We should start supporting such a scenario also.