Allow to override the gitlab-qa ref for the downstream trigger

What does this MR do?

This allows to override the gitlab-qa ref for the downstream trigger.

This supports gitlab-qa!191 (merged). Needs omnibus-gitlab!2797 (merged).

What are the relevant issue numbers?

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/47818.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides