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
diff --git a/tests/fixtures/layout.yaml b/tests/fixtures/layout.yaml
index 2695719..37ea552 100644
--- a/tests/fixtures/layout.yaml
+++ b/tests/fixtures/layout.yaml
@@ -31,6 +31,7 @@
       verified: -2
     start:
       verified: 0
+    precedence: high
 
   - name: unused
     manager: IndependentPipelineManager