s3proxy should use its own IAM config instead of reusing the incoming client credentials

[benmcclelland]

It would be better to just have the s3proxy use a single account on the backend instead of reusing the incoming credentials. This way the admin can have a single backend s3 account, and provide internal access through local IAM management.

The changes will need to be that S3be should init with its own s3access/s3secret that get configured from cli or environment. And the S3 actions should use this account for backend access instead of the stored credentials in the context.