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/zuul/model.py b/zuul/model.py
index a7c7f0d..acefef6 100644
--- a/zuul/model.py
+++ b/zuul/model.py
@@ -28,6 +28,8 @@
     def __init__(self, name):
         self.name = name
         self.description = None
+        self.failure_message = None
+        self.success_message = None
         self.job_trees = {}  # project -> JobTree
         self.manager = None
         self.queues = []