Remove auto creation of bucket for s3 repository

[rjernst]

Creating a bucket is a management operation which a user should have already done, and only needs to do once. We should remove auto bucket creation and error if the configured bucket does not exist.

[dadoonet]

I agree.

[dadoonet]

Discussed on FixIt friday session. We agreed that we should remove auto creation of S3 bucket and fails if it not exists.