Use multi-project pipelines for RAT triggers
We used custom trigger token for RAT because we didn't want to create RAT-mirror
just to let developers run pipeline against master
branch. However, on re-thinking, I think we can open up the master
branch of RAT for developers to merge and just enforce MR approvals from maintainers - so that multi-project pipelines can be used. RAT is not as critical or user-facing as omnibus-gitlab (where we went with a separate mirror because we didn't want to open up master
branch to developers), so it should be relatively safe to do it.
Note: Nightlies and tags will still use the token because they are triggered from dev.gitlab.org to gitlab.com.
Ideally gitlab#24585 should be done so that ability to run pipelines against a branch is separated from ability to push/merge to it.