Add support for encrypted data stored in S3

Add support for client-side encryption of notebooks stored in Amazon S3. Right now, notebooks are stored in plaintext and for our use of Zeppelin we need to encrypt this data.

I have been using a modified version of Zeppelin locally for some time which adds support for encrypting notebooks stored in S3. These changes are to ZeppelinConfiguration.java and S3NotebookRepo.java and support the following:

• Use the AWS KMS to encrypt data stored in S3
• (or) Use a custom S3 EncryptionMaterialsProvider class to encrypt data in S3
• Minor improvements to logic in the S3 notebook repo when creating local files, etc.