Ignore errors when deleting tags from dockerhub

We've seen a case where we can still push and pull tags from dockerhub,
but the web UI and API seem out of sync with the actual registry.  In
this case, we would like to continue, even though it will leave some
unused tags in the repo (they can be cleaned up later if they ever
show up).

Change-Id: If000163a321c869c46cfed4233c2ea42c3e8471b

roles/promote-docker-image/tasks/promote-retag-inner.yaml

@@ -23,6 +23,9 @@
       Authorization: "Bearer {{ token.json.token }}"
 - name: Delete the current change tag
   no_log: true
+  # In case the dockerhub API is out of sync with the registry, let's
+  # continue anyway.
+  failed_when: false
   uri:
     url: "https://hub.docker.com/v2/repositories/{{ zj_image.repository }}/tags/{{ promote_tag_prefix }}_{{ zj_image_tag }}/"
     method: DELETE