Specify a default for container_registry_logins

When we run this, we'll run the controller host_prep_tasks but the
compute nodes are included. Ansible will skip the set_fact on the
computes but still try to evaluate the lookup. We need to use |default
to handle the unset case.

Change-Id: I1ff41befbd66325d90c3ea18640a298549d1b659
Closes-Bug: #1835657
(cherry picked from commit a6ff00885c)

deployment/podman/podman-baremetal-ansible.yaml

@@ -111,7 +111,7 @@ outputs:
               REGISTRY_USERNAME: "{{ lookup('dict', item.value).key }}"
               REGISTRY_PASSWORD: "{{ lookup('dict', item.value).value }}"
               REGISTRY: "{{ item.key }}"
-            loop: "{{ query('dict', container_registry_logins) }}"
+            loop: "{{ query('dict', container_registry_logins | default({})) }}"
             when:
               - container_registry_login | bool
               - container_registry_logins