Add support for Docker plugins (#4934)

* Add support for Docker plugins

* support multiple Docker plugins using looped include

* fix yamllint error

docs/vars.md

@@ -98,6 +98,7 @@ Stack](https://github.com/kubernetes-sigs/kubespray/blob/master/docs/dns-stack.m
 
 * *docker_options* - Commonly used to set
   ``--insecure-registry=myregistry.mydomain:5000``
+* *docker_plugins* - This list can be used to define [Docker plugins](https://docs.docker.com/engine/extend/) to install. 
 * *http_proxy/https_proxy/no_proxy* - Proxy variables for deploying behind a
   proxy. Note that no_proxy defaults to all internal cluster IPs and hostnames
   that correspond to each node.

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

+---
+- 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
 

roles/kubespray-defaults/defaults/main.yaml

@@ -256,6 +256,10 @@ docker_options: >-
   --userland-proxy-path=/usr/libexec/docker/docker-proxy-current --signature-verification=false
   {%- endif -%}
 
+## A list of plugins to install using 'docker plugin install --grant-all-permissions'
+## Empty by default so no plugins will be installed.
+docker_plugins: []
+
 # Experimental kubeadm etcd deployment mode. Available only for new deployment
 etcd_kubeadm_enabled: false