Restrict use of AmazonS3.listBuckets()

[sbeimin]

We're running into a snag because the access key we have for our bucket is not authorized to do the ListAllMyBuckets call. This is a security decision since ListAllMyBuckets returns all buckets instead of just the bucket our access key is authorized to use.

S3FileStore.getBucket() uses the listBuckets() to get the Bucket object. Where possible this should be avoided.