Explicitly store date facts for promote

It would be convenient to use date when promoting an image.
This gives the opportunity to freeze an image on dockerhub for
example, and keep an always rolling image at the same time.

Without this patch, ansible_date_time is not exposed on the
runner, which prevents its use.

Change-Id: I8798c64e8516a45cc87ad2e3c082edc5e3e1f044

playbooks/docker-image/promote.yaml

@@ -1,3 +1,9 @@
 - hosts: localhost
+  # The gather_facts need to stay on smart (default on zuul executor)
+  # to keep the fact gathered to a minimum.
+  pre_tasks:
+    - name: Gather minimal set of facts
+      setup:
+        filter: ansible_date_time
   roles:
     - promote-docker-image