Use external docker image with instance tests to run QA pipeline

After #30 (closed) and https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1881 are done, update GitLab QA standalone tool to work with the new source of docker images containing instance specs.

mentioned in issue #32 (closed)

mentioned in issue #25 (closed)

assigned to @grzesiek

changed milestone to %9.0

mentioned in merge request !58 (merged)

closed via merge request !58 (merged)

closed via commit c4aa7a45

mentioned in commit c4aa7a45