CI: Remove unused Docker images after overcloud upgrade

Overcloud upgrade jobs often fail when running on nodes with a 38G root
disk. The overcloud upgrade itself generally works, but post-upgrade
testing fails, for example when RabbitMQ gets in alarm state due to lack
of free disk space, which prevents Nova from functioning correctly and
makes it return "Unknown Error (HTTP 504)".

Change-Id: I536a526340f6d7dea7a69372904b777292774eba

playbooks/kayobe-overcloud-upgrade-base/run.yml

@@ -81,6 +81,12 @@
           shell:
             cmd: "{{ kayobe_src_dir }}/dev/overcloud-upgrade.sh &> {{ logs_dir }}/ansible/overcloud-upgrade"
 
+        # Remove unused Docker images to avoid reaching full disk
+        - name: Remove unused Docker images
+          shell:
+            cmd: "docker image prune --all --force"
+          become: true
+
         # Perform a smoke test against the upgraded current release.
 
         - name: Perform testing of VMs in the upgraded overcloud