Use child pipelines instead of multi-project pipelines for package-and-qa runs

Now with the ability to run any type of pipelines in any mirror of the project, for MRs from omnibus-gitlab, instead of triggering a pipeline in omnibus-gitlab-mirror, we can just create a child pipeline. This will save us from "waiting for SHA in mirror" problems during a security release.

Note: For package-and-qa runs from gitlab-org/gitlab MRs, we will still use omnibus-gitlab-mirror so as not to pollute omnibus-gitlab pipelines page with such triggers.