MongoNotebookRepo get unstable note path in zeppelin 0.9.0-preview1

In 0.9.0, when using MongoNotebookRepo, zeppelin store note paths in folder collection and get note paths by aggregation $graphLookup.

However, when I create notes in multi-layer folders, and then restart zeppelin or call /api/notebook-repositories/reload, the path of note is unstable.

The order of documents returned by $graphLookup leads to this problem. I investigate $graphLookup through mongodb manual(https://docs.mongodb.com/v3.4/reference/operator/aggregation/graphLookup/).The reference indicates that documents returned in the as field are not guaranteed to be in any order. Then I view the source of MongoNotebookRepo, and I found that zeppelin builds the note path without sorting the documenent returned in as field of $graphLookup. I'll be happy to submit a PR to fix this if you like.