Only run checks on open patches

We don't want 'recheck' comments to trigger checks on merged patches.

Change-Id: I0b4a56e86563fd47a7a147aa920dcd147ca3548e

zuul.d/pipelines.yaml

@@ -11,6 +11,10 @@
       Build failed (check pipeline).  For information on how to proceed, see
       http://docs.openstack.org/infra/manual/developers.html#automated-testing
     manager: independent
+    require:
+      gerrit:
+        open: True
+        current-patchset: True
     trigger:
       gerrit:
         - event: patchset-created