(zuul) Fix image publish job

- Fix issue in post pipeline image publish job introduced
  by Ansible update

Change-Id: Ia97f2927980f5ee5c2d5adf23a5a016b8d3b1c9f

tools/gate/roles/build-images/tasks/airship-shipyard.yaml

@@ -26,10 +26,10 @@
 - name: Determine tags
   shell: echo '{{ tags | to_json }}' | python {{ zuul.project.src_dir }}/tools/image_tags.py
   environment:
-    BRANCH: "{{ zuul.branch }}"
-    CHANGE: "{{ zuul.change }}"
+    BRANCH: "{{ zuul.branch | default('') }}"
+    CHANGE: "{{ zuul.change | default('') }}"
     COMMIT: "{{ zuul.newrev | default('') }}"
-    PATCHSET: "{{ zuul.patchset }}"
+    PATCHSET: "{{ zuul.patchset | default('') }}"
   register: image_tags
 
 - name: Debug computed tags