S3DataStore should use virtual-host-style access

AWS has announced that they are moving from a path-based model for addressable storage to a virtual-host model (see [0]).  S3DataStore is implemented using the path-based model, and should update to use the virtual-host model before the old format is deprecated.

[0] - https://aws.amazon.com/blogs/aws/amazon-s3-path-deprecation-plan-the-rest-of-the-story/