Consider renaming the gitlab-qa Docker image

As you can see in the GitLab QA Architecture diagram gitlab-qa is very misleading:

1. The gitlab-qa gem is an orchestration tool
2. We publish the the gitlab/gitlab-qa Docker images that are built from https://gitlab.com/gitlab-org/gitlab-ce/tree/master/qa

I think gitlab-qa for the gem makes total sense so I suggest we rename the Docker images to gitlab-ce-qa/gitlab-ee-qa (initially I started doing it in gitlab-org/omnibus-gitlab!2028 (merged) but that was out of scope).

/cc @grzesiek @balasankarc @richardc