Use external docker image with instance tests to run QA pipeline
After #30 (closed) and omnibus-gitlab#1881 (closed) are done, update GitLab QA standalone tool to work with the new source of docker images containing instance specs.
After #30 (closed) and omnibus-gitlab#1881 (closed) are done, update GitLab QA standalone tool to work with the new source of docker images containing instance specs.