chore(modules/storage): make process launcher callback optional

We aren't always interested in calling a function after the storage
process has been launched, so passing a callback should be optional.

lib/modules/storage/storageProcessesLauncher.js

@@ -86,6 +86,8 @@ class StorageProcessesLauncher {
 
   launchProcess(storageName, callback) {
     const self = this;
+    callback = callback || function () {};
+
     if (self.processes[storageName]) {
       return callback(__('Storage process already started'));
     }