CI: Ensure docker-py is installed on publish

After a recent change [1] publish jobs are failing on missing docker python
SDK.

[1]: https://review.opendev.org/c/openstack/kolla/+/763146

Change-Id: I63662dd903b914fe06165b45e0a0bb5ca74b21b9

tests/playbooks/publish.yml

@@ -1,6 +1,12 @@
 ---
 - hosts: all
   tasks:
+    - name: Ensure docker python SDK is installed
+      become: true
+      pip:
+        name: "docker"
+        state: present
+
     - name: List all containers
       docker_host_info:
         images: yes