Switch off of docker.io for testing

Let's use the ubi8 container instead of centos:8

Change-Id: I5985c8c3db90024a99e1d55d466b46076be6857a
(cherry picked from commit 83706a92c5)

tripleo_ansible/roles/tripleo_podman/molecule/default/prepare.yml

@@ -36,12 +36,12 @@
   post_tasks:
     - name: pull an image
       podman_image:
-        name: centos:8
+        name: "ubi8:latest"
 
     - name: Create a data container
       podman_container:
         name: "{{ item }}"
-        image: centos:8
+        image: "ubi8:latest"
         detach: true
         command: sleep 1d
       with_items: