Ensure old installation match new images and get python-docker.

If user is coming from a <=Newton version, then it may be missing that
package. Make sure it's installed before during update or upgrade.

Change-Id: If0ae6c65468ef19183419261781745fafb56c9b7
Closes-Bug: #1917913

puppet/services/tripleo-packages.yaml

@@ -300,6 +300,13 @@ outputs:
             name: '*'
             state: latest
             exclude: ceph*,librados*,librbd*,libcephfs*,librgw*,python-rados*,python-rbd*,python-cephfs*,python-rgw*,rbd-mirror
+        - name: Make sure python-docker is installed.
+          when:
+            - step|int == 3
+            - not skip_package_update|bool
+          yum:
+            name: 'python-docker'
+            state: 'installed'
       update_tasks:
         # With the layered product packaging, the key package is rhosp-openvswitch. It depends on
         # a openvswitch package that includes the version as part of the name (e.g openvswitch2.10).
@@ -423,6 +430,13 @@ outputs:
             name: '*'
             state: latest
             update_cache: yes
+        - name: Make sure python-docker is installed.
+          when:
+            - step|int == 3
+            - not skip_package_update|bool
+          yum:
+            name: 'python-docker'
+            state: 'installed'
         # This is failsafe unless openvswitch package does something to the systemd service state.
         - name: Ensure openvswitch is running after update
           when: