builder/amazon: Fix S3 endpoint URL for instance-store builds

The S3 endpoint URLs don't always include the region name. See:

    http://docs.aws.amazon.com/general/latest/gr/rande.html#s3_region

This mapping is already provided by `goamz` via `region.S3Endpoint`,
so let's use it.
Merge pull request #904 from grosskur/fix-s3-endpoint

builder/amazon: Fix S3 endpoint URL for instance-store builds