Add option to ignore ref-updated events emitted by branch deletions

When a branch is deleted, gerrit emits a ref-updated event with a newrev
value of all zeros. This adds a boolean field to optionally not trigger
on these ref-updated events.

Change-Id: Iea75d05ddcb49b0bf748b72b9d2d5472d077f0c6
diff --git a/etc/layout.yaml-sample b/etc/layout.yaml-sample
index 30a3352..53f6ba1 100644
--- a/etc/layout.yaml-sample
+++ b/etc/layout.yaml-sample
@@ -30,6 +30,7 @@
       gerrit:
         - event: ref-updated
           ref: ^(?!refs/).*$
+          ignore-deletes: False
 
   - name: gate
     manager: DependentPipelineManager