Added silent start mode for gating jobs

Change-Id: I3300397a0a0a2f5406336915bcca927555599201
diff --git a/jenkins/client/job/gating.yml b/jenkins/client/job/gating.yml
index d0f2149..6886b06 100644
--- a/jenkins/client/job/gating.yml
+++ b/jenkins/client/job/gating.yml
@@ -38,6 +38,7 @@
             quiet_period: 60
             trigger:
               gerrit:
+                silent_start_mode: true
                 project:
                   '{{regex}}':
                     compare_type: REG_EXP