[server/uiSettings] add enabled config

in response to matt's comment that relying on `plugins.initialize` is kinda dirty (with which we all agree) we're adding an undocumented config value that the dev cli can use to put prevent the optimizr server from logging meaningless warnings

src/cli/cluster/cluster_manager.js

@@ -18,6 +18,7 @@ module.exports = class ClusterManager {
     const serverArgv = [];
     const optimizerArgv = [
       '--plugins.initialize=false',
+      '--uiSettings.enabled=false',
       '--server.autoListen=false',
     ];
 

src/server/config/schema.js

@@ -145,6 +145,10 @@ module.exports = () => Joi.object({
       reuseTiles: Joi.boolean(),
       bounds: Joi.array().items(Joi.array().items(Joi.number()).min(2).required()).min(2)
     }).default()
-  }).default()
+  }).default(),
+
+  uiSettings: Joi.object({
+    enabled: Joi.boolean().default(true)
+  }).default(),
 
 }).default();

src/ui/settings/index.js

@@ -4,8 +4,8 @@ import defaultsProvider from './defaults';
 export default function setupSettings(kbnServer, server, config) {
   const status = kbnServer.status.create('ui settings');
 
-  if (!kbnServer.config.get('plugins.initialize')) {
-    status.disabled('Disabled because plugins are disabled');
+  if (!kbnServer.config.get('uiSettings.enabled')) {
+    status.disabled('uiSettings.enabled config is set to `false`');
     return;
   }