Set build update log level to debug

We get this a lot.  It's not helpful.

Change-Id: Icf0191ded019c705a42446269d647cb52843ef58
Reviewed-on: https://review.openstack.org/36733
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins
diff --git a/zuul/launcher/gearman.py b/zuul/launcher/gearman.py
index 0f9cc54..4ca7c27 100644
--- a/zuul/launcher/gearman.py
+++ b/zuul/launcher/gearman.py
@@ -370,7 +370,7 @@
 
     def onWorkStatus(self, job):
         data = getJobData(job)
-        self.log.info("Build %s update %s " % (job, data))
+        self.log.debug("Build %s update %s " % (job, data))
         build = self.builds.get(job.unique)
         if build:
             self.log.debug("Found build %s" % build)