Create gitlab-qa-mirror project and use multi-project pipelines to trigger pipelines there from omnibus-gitlab-mirror

Spinoff from omnibus-gitlab#4717 (closed).

A project omnibus-gitlab-mirror was created to make use of multi-project pipelines for package-and-qa jobs. In this project, master branch was opened to all Developers of gitlab-org to merge (but explicitly disabled MRs from project settings so that no one actually does). Doing this enabled them to run pipeline against master branch, which meant we could use multi-project pipelines for triggering jobs instead of using a specific bot user's token.

This issue is to track implementing the same thing in gitlab-qa, using a mirror called gitlab-qa-mirror. It also makes sense to tackle #261 (closed) along with it.