buildset-registry: add flag to make job fail

Add a flag that can be set to make this job fail, which can be useful
for intsructing Zuul to hold the node if debugging failures.

Change-Id: I96123c3f585a59958932e9a8cab6aa3b685764d7

playbooks/buildset-registry/post.yaml

@@ -20,3 +20,10 @@
       shell: "docker logs buildset_registry &> {{ ansible_user_dir }}/zuul-output/logs/docker/buildset_registry.txt"
       args:
         executable: /bin/bash
+
+    # This can be useful to make sure you can put a hold on the
+    # registry node if you need to debug it
+    - name: Trigger failure if required
+      fail:
+        msg: 'Triggering failure for debugging'
+      when: buildset_registry_debug_fail|default(false)|bool