Support Ceph as a notebook storage

Make Zeppelin support Ceph as a notebook storage.

Ceph has APIs which are compatible with AWS S3 APIs. However, it supports only AWS Signature V2 and GetObject requests of aws-sdk-java use V4 by default: https://github.com/aws/aws-sdk-java/issues/372

Zeppelin configuration which enforces an S3 client to use AWS Signature V2 is necessary for Zepplin to work with Ceph successfully.