Add support for Docker plugins (#4934)

* Add support for Docker plugins

* support multiple Docker plugins using looped include

* fix yamllint error

roles/container-engine/docker/tasks/docker_plugin.yml

@@ -0,0 +1,8 @@
+---
+- name: Install Docker plugin
+  command: docker plugin install --grant-all-permissions {{ docker_plugin | quote }}
+  when: docker_plugin is defined
+  register: docker_plugin_status
+  failed_when:
+    - docker_plugin_status.failed
+    - '"already exists" not in docker_plugin_status.stderr'

roles/container-engine/docker/tasks/main.yml

@@ -235,6 +235,12 @@
         resolvconf_mode == 'docker_dns' and
         installed_docker_version.stdout is version('1.12', '<')
 
+# Install each plugin using a looped include to make error handling in the included task simpler.
+- include_tasks: docker_plugin.yml
+  loop: "{{ docker_plugins }}"
+  loop_control:
+    loop_var: docker_plugin
+
 - name: Set docker systemd config
   import_tasks: systemd.yml