Build Gerrit branches automatically

The Gerrit-verifier-pipeline is now responsible to
build only the changes that are discovered by the
Checks API. The stable branches need a different way
to get triggered: use the standard pollscm configuration
to make sure that they are checked and rebuilt.

Change-Id: Ieb07c8428291a59d224a8fa3afc7bd50872ed4c9
diff --git a/jenkins/gerrit-bazel-template.yaml b/jenkins/gerrit-bazel-template.yaml
index 0ee3b1c..71824a7 100644
--- a/jenkins/gerrit-bazel-template.yaml
+++ b/jenkins/gerrit-bazel-template.yaml
@@ -19,6 +19,9 @@
             recursive: true
           skip-tag: true
           basedir: gerrit
+    triggers:
+      - pollscm:
+          cron: 'H/50 * * * *'
     wrappers:
       - timeout:
           timeout: 90