refactor client creation into its own function

This will allows to easily create multiple clients later.

app.js

@@ -30,10 +30,16 @@ console.log('starting hublot...');
 controller.loadAll('controller', 'lib', 'robot')
   .then(modules => {
     console.log('modules loaded... launching runner');
+
+    // Note: client returned object can be used to control further the browser
+    // e.g.: let c =  client('room'); c.end();
+    const client = room => runner.run(modules,
+                                      config.visio.url,
+                                      room,
+                                      config.client);
+
     const room = 'test-bot';
-    // Note: client can be used to control further the browser
-    // e.g.: let client =  runner.run(...); client.end();
-    registry[room] = runner.run(modules, config.visio.url, room, config.client);
+    registry[room] = client(room);
   })
   .catch(err => {
     console.error(err);