Make report message introduction configurable.

By setting parameters on a pipeline, you can change the default
"Build successful/failed" introductory messages left on Gerrit.

Change-Id: Ie05c8c87a47994faced4ee67fda4ab4e64ea0d20
Reviewed-on: https://review.openstack.org/19677
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 cfe68fc..586e2a5 100644
--- a/tests/fixtures/layout.yaml
+++ b/tests/fixtures/layout.yaml
@@ -16,6 +16,7 @@
 
   - name: gate
     manager: DependentPipelineManager
+    failure-message: Build failed.  For information on how to proceed, see http://wiki.example.org/Test_Failures
     trigger:
       - event: comment-added
         approval: