Add pipeline precedence

Allow, eg, jobs in a gate pipeline to take precedence over
a check pipeline.

Change-Id: Idf91527704cc75b00a336291f91b908286f8e630
Reviewed-on: https://review.openstack.org/36552
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins