Allow to use any gitlab-qa image tag

Right now, staging.gitlab.com is running 10.1.3 so I'd like to run QA against it with the following command:

GITLAB_USERNAME=gitlab-qa GITLAB_PASSWORD="REDACTED" bin/qa Test::Instance::Any EE 10.1.3-ee.0 https://staging.gitlab.com

which would pull the 10.1.3-ee.0 image from https://hub.docker.com/r/gitlab/gitlab-qa/tags/.

Unfortunately, the current codebase forces the gitlab-qa that is used to pull the <edition>-<tag> tag, which would mean ee-10.1.3-ee.0 in this case, but it doesn't exist.

We could solve it in two ways:

1. Use the tag given as-is: that would mean that to use the latest EE gitlab-qa image, you'd call bin/qa Test::Instance::Any EE ee-latest https://staging.gitlab.com
2. Fix the naming in Omnibus to generate <edition>-<tag> image tags.

/cc @grzesiek