Using two notebook storage classes - second repo's notes are not synced when server starts

How to reproduce (I tested with S3 repo as the second storage):

1. Have a Zeppelin S3 repo with pre-existing notes
2. Set the "zeppelin.notebook.storage" setting set to two classes - the first one can be any class and the second one the S3 class. eg: "org.apache.zeppelin.notebook.repo.GitNotebookRepo,org.apache.zeppelin.notebook.repo.S3NotebookRepo"
   (other settings are also required for the S3 repo)
3. Set the "zeppelin.anonymous.allowed" setting to false and use shiro to set up some user
4. Start Zeppelin and only the first repo's notes are loaded
5. If you click the "Reload notes from storage" button notes from the second repo will be loaded as well

If anonymous access is allowed, notes from the second repo will be loaded as soon as Zeppelin starts due to a condition at the bottom of NotebookRepoSync's constructor:

    // sync for anonymous mode on start
    if (getRepoCount() > 1 && conf.getBoolean(ConfVars.ZEPPELIN_ANONYMOUS_ALLOWED)) {
      try {
        sync(AuthenticationInfo.ANONYMOUS);
      } catch (IOException e) {
        LOG.error("Couldn't sync on start ", e);
      }
    }