Define a release-approval pipeline to run the check-release-approval
job on every comment added to a release request, and set a
PTL-Approved label accordingly.
This may be considered a bit resource-intensive, however the
check-release-approval job is a fast python script that runs on
the executor, and only release requests shall go in this pipeline.
If this generates too much load, we could configure it to only run
when the comment posted contains a magic "signoff" keyword.
Another concern is that jobs other than check-release-approval would
be added to this pipeline. There does not seem to be a way in Zuul to
limit a pipeline to a specific job name or project.
Change-Id: Ieab04a4d6c02b216a59c12ec8599e7d91f4fffb1
7e896b57eb