Updating Docker Gate use of zuul.newrev

- Zuul updated ansible to 2.7, no longer allows missing variables.
- Using default value when it isn't available.

Based on Aaron Sheffield's PS for Pegleg: https://review.openstack.org/#/c/645631/

Change-Id: I02495bc793021b429e0be62ecac0ed45e930484c

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

@@ -28,7 +28,7 @@
   environment:
     BRANCH: "{{ zuul.branch }}"
     CHANGE: "{{ zuul.change }}"
-    COMMIT: "{{ zuul.newrev }}"
+    COMMIT: "{{ zuul.newrev | default('') }}"
     PATCHSET: "{{ zuul.patchset }}"
   register: image_tags
 
@@ -111,7 +111,7 @@
           DOCKER_REGISTRY: "quay.io"
           IMAGE_PREFIX: "airshipit"
           IMAGE_TAG: "{{ item }}"
-          COMMIT: "{{ zuul.newrev }}"
+          COMMIT: "{{ zuul.newrev | default('') }}"
           PUSH_IMAGE: "true"
       with_items: "{{ image_tags.stdout_lines }}"