Adds Docker image pruning command

This change introduces prune-images command.

Uses docker_prune module of Ansible that comes with version 2.8.

Depends-On: https://review.opendev.org/#/c/699333/

Implements: blueprint docker-image-pruning

Change-Id: Icbf374dd50e1cc1f1604bb4fa779b34279efd50c

ansible/prune-images.yml

+---
+- import_playbook: gather-facts.yml
+
+- name: Apply role prune-images
+  hosts: baremetal
+  serial: '{{ kolla_serial|default("0") }}'
+  gather_facts: false
+  roles:
+    - prune-images

ansible/roles/prune-images/defaults/main.yml

+---
+docker_image_prune_timeout: 300

ansible/roles/prune-images/tasks/main.yml

+---
+- include_tasks: validate_docker_execute.yml
+
+- include_tasks: prune_images.yml

ansible/roles/prune-images/tasks/prune_images.yml

+---
+- name: Pruning Kolla images
+  become: true
+  docker_prune:
+    images: yes
+    images_filters:
+      label: kolla_version
+    timeout: "{{ docker_image_prune_timeout }}"

ansible/roles/prune-images/tasks/validate_docker_execute.yml

+---
+- name: Ensure the docker service is running
+  script: ../tools/validate-docker-execute.sh

doc/source/user/operating-kolla.rst

@@ -191,6 +191,9 @@ tests.
 ``kolla-ansible -i INVENTORY deploy-containers`` is used to check and if
 necessary update containers, without generating configuration.
 
+``kolla-ansible -i INVENTORY prune-images`` is used to prune orphaned Docker
+images on hosts.
+
 .. note::
 
    In order to do smoke tests, requires ``kolla_enable_sanity_checks=yes``.

releasenotes/notes/docker-image-pruning-fffcffa7e99c6e63.yaml

+---
+features:
+  - |
+    Adds command ``prune-images`` for Docker image pruning on hosts. See
+    `blueprint
+    <https://blueprints.launchpad.net/kolla-ansible/+spec/docker-image-pruning>`__
+    for details.

tools/kolla-ansible

@@ -88,6 +88,7 @@ Commands:
     upgrade             Upgrades existing OpenStack Environment
     upgrade-bifrost     Upgrades an existing bifrost container
     genconfig           Generate configuration files for enabled OpenStack services
+    prune-images        Prune orphaned Kolla images
 EOF
 }
 
@@ -126,6 +127,7 @@ certificates
 upgrade
 upgrade-bifrost
 genconfig
+prune-images
 EOF
 }
 
@@ -391,6 +393,18 @@ EOF
         ACTION="Generate configuration files for enabled OpenStack services"
         EXTRA_OPTS="$EXTRA_OPTS -e kolla_action=config"
         ;;
+(prune-images)
+        ACTION="Prune orphaned Kolla images"
+        PLAYBOOK="${BASEDIR}/ansible/prune-images.yml"
+        if [[ "${DANGER_CONFIRM}" != "--yes-i-really-really-mean-it" ]]; then
+            cat << EOF
+WARNING:
+    This will PERMANENTLY DELETE all orphaned kolla images. To confirm, please add the following option:
+    --yes-i-really-really-mean-it
+EOF
+            exit 1
+        fi
+        ;;
 (bash-completion)
         bash_completion
         exit 0