Updating Docker Gate use of zuul.newrev

- Zuul updated ansible to 2.7, no longer uses missing variables.
- Using an if to try and address.

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

Change-Id: I6cf52a2b9c804c29cc727ad60d45c05a8450c9e9

tools/zuul/playbooks/docker-image-build.yaml

@@ -62,7 +62,7 @@
       environment:
         BRANCH: "{{ zuul.branch }}"
         CHANGE: "{{ zuul.change }}"
-        COMMIT: "{{ zuul.newrev }}"
+        COMMIT: "{{ zuul.newrev | default('') }}"
         PATCHSET: "{{ zuul.patchset }}"
       register: image_tags
 
@@ -102,7 +102,7 @@
               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 }}"