Restructre clusterctl cli interface

Turn the flat list of commands into a hierarchy of commands, converting
most required parameters into positional parameters and add plugin
loading support for clusterctl.

lib/plugin.js

@@ -11,11 +11,26 @@ const errors = require("lib/errors");
 
 /**
  * Conceptual base for master and instance plugins.
- * @typedef {module:lib/plugin.BaseMasterPlugin|module:lib/plugin.BaseInstancePlugin} module:lib/plugin.BasePlugin
+ * @typedef {
+ *     module:lib/plugin.BaseMasterPlugin
+ *     |module:lib/plugin.BaseInstancePlugin
+ *     |module:lib/plugin.BaseControlPlugin
+ * } module:lib/plugin.BasePlugin
  */
 
 /**
  * Base class for instance plugins
+ *
+ * Instance plugins are subclasses of this class which get instantiated by
+ * the slave when it brings up an instance with the plugin enabled in the
+ * config.  To be discovered the class must be exported under the name
+ * `InstancePlugin` in the module specified by the `instanceEntrypoint` in
+ * the plugin's info.js file.
+ *
+ * Instances may be started and stopped many times, and many instances may
+ * be running at the same time, each of which will have their own instance
+ * of the InstancePlugin class.
+ *
  * @static
  */
 class BaseInstancePlugin {
@@ -185,6 +200,13 @@ class BaseInstancePlugin {
 
 /**
  * Base class for master plugins
+ *
+ * Master plugins are subclasses of this class which get instantiated by
+ * the master server on startup when the plugin is enabled in the config.
+ * To be discovered the class must be exported under the name `MasterPlugin`
+ * in the module specified by the `masterEntrypoint` in the plugin's info.js
+ * file.
+ *
  * @static
  */
 class BaseMasterPlugin {
@@ -316,6 +338,42 @@ class BaseMasterPlugin {
 	}
 }
 
+/**
+ * Base class for clusterctl plugins
+ *
+ * Control plugins are subclasses of this class which get instantiated by
+ * clusterctl in order to extend its functionallity.  To be discovered the
+ * class must be exported under the name `ControlPlugin` in the module
+ * specified by the `controlEntrypoint` in the plugin's info.js file.
+ *
+ * @static
+ */
+class BaseControlPlugin {
+	constructor(info) {
+		/**
+		 * The plugin's own info module
+		 */
+		this.info = info;
+	}
+
+	/**
+	 * Called immediately after the class is instantiated
+	 */
+	async init() { }
+
+	/**
+	 * Called to add commands to the command line interface.
+	 *
+	 * Invoked by clusterctl to let plugins add commands.  `rootCommand` is
+	 * the top level command node which the plugin should add its own {@link
+	 * module:lib/command.CommandTree} to.
+	 *
+	 * @param {module:lib/command.CommandTree} rootCommand -
+	 *     Root of the clusterctl command tree.
+	 */
+	async addCommands(rootCommand) { }
+}
+
 /**
  * Load plugin information
  *
@@ -413,6 +471,7 @@ async function invokeHook(plugins, hook, ...args) {
 module.exports = {
 	BaseInstancePlugin,
 	BaseMasterPlugin,
+	BaseControlPlugin,
 
 	loadPluginInfos,
 	attachPluginMessages,