Allow Instance Profile Credentials to be used for S3 uploads/downloads

This is an updated version of the patches from https://github.com/jenkinsci/s3-plugin/pull/9 against the latest s3-plugin.

Tested in my environment with and without Instance Profile Credentials:
- confirmed to work in both cases (IAM Role and User with password and key)
- confirmed to fail as expected when no Role Policy allows access to the buckets :)