Check if docker enabled before stopping all containers.

The set of tasks in tripleo-podman/tasks/tripleo_docker_stop.yml
are being used during the upgrade to ensure that all
services are stopped when upgrading from Rocky to Stein.
However, in Train docker does not have to be installed in
the system and this task would fail.
Adding a check before to stop the services only if docker
is installed.

Change-Id: I05fa46d3b53ece83867a2ccd5c931b0cac1209d4

tripleo_ansible/roles/tripleo-podman/tasks/tripleo_docker_stop.yml

@@ -21,12 +21,20 @@
     - system_upgrade_prepare
   become: true
   block:
+    - name: Check if docker is enabled in the system
+      stat:
+        path: "/var/run/docker.sock"
+      register: check_docker
+      failed_when: false
+
     - name: Stop all services by stopping all Docker containers
       command: docker ps -aq
       register: running_containers
+      when: check_docker.stat.exists
 
     - name: Stop a container
       docker_container:
         name: "{{ item }}"
         state: stopped
       loop: "{{ running_containers.stdout_lines | default([]) }}"
+      when: check_docker.stat.exists