Doesn't sync with secondary storage while running (need restart)

Previously Zeppelin have been loading all the notebooks from storage to memory at the start and wouldn't detect if new notebook folders were manually added into ZEPPELIN_NOTEBOOK_DIR. After https://github.com/apache/incubator-zeppelin/pull/331 in-memory notebooks will be updated on every listing if ZEPPELIN_NOTEBOOK_RELOAD_FROM_STORAGE is true. However this new notebooks will not be synced with secondary storage if one is set ( https://github.com/apache/incubator-zeppelin/pull/123 ).