Do not publish images on docker hub

The image promotion job often fails
due to Docker hub rate limits. The promotion
is run in the promote pipeline which starts
after the change is merged and we can not
use `recheck` to re-run the job. Let's
stop trying to publish images to Docker hub
and only publish them to quay.io (which is the
current default source of images).

Change-Id: Ife7ff860c465b3bbb20c437125b5e091619f67c0

playbooks/vars.yaml

@@ -2,8 +2,6 @@ currentdate: "{{ now(utc=True,fmt='%Y%m%d') }}"
 local_registry: "172.17.0.1:5000"
 local_registry_image: "quay.io/airshipit/registry:2"
 image_registries:
-  - host: "docker.io"
-    org: "loci"
   - host: "quay.io"
     org: "airshipit"
 container_command: docker