Make check, gate, post low precedence

To give check a chance to catch up to gate.

Change-Id: I88690cd3f2dc402e3b963596a71554914c0611b5

modules/openstack_project/files/zuul/layout.yaml

@@ -6,7 +6,7 @@ pipelines:
     description: Newly uploaded patchsets enter this pipeline to receive an initial +/-1 Verified vote from Jenkins.
     failure-message: Build failed.  For information on how to proceed, see https://wiki.openstack.org/wiki/GerritJenkinsGit#Test_Failures
     manager: IndependentPipelineManager
-    precedence: high
+    precedence: low
     trigger:
       gerrit:
         - event: patchset-created
@@ -23,7 +23,7 @@ pipelines:
     description: Changes that have been approved by core developers are enqueued in order in this pipeline, and if they pass tests in Jenkins, will be merged.
     failure-message: Build failed.  For information on how to proceed, see https://wiki.openstack.org/wiki/GerritJenkinsGit#Test_Failures
     manager: DependentPipelineManager
-    precedence: high
+    precedence: low
     trigger:
       gerrit:
         - event: comment-added
@@ -45,6 +45,7 @@ pipelines:
   - name: post
     description: This pipeline runs jobs that operate after each change is merged.
     manager: IndependentPipelineManager
+    precedence: low
     trigger:
       gerrit:
         - event: ref-updated