packer: Delete "override" key for config passed to provisioners

The overrides are processed, so that the provisioner only sees the
appropriate sub-settings, so the provisioner does not need the raw
overrides.

If we leave the top-level overrides object in the provisioner
configuration, then it will be rejected as an unknown configuration
key.

[GH-152]

Currently the passed in AWS auth or AWS environment variables are
interpreted by packer. This change moves that logic into goamz in
order to support both the existing and instance based IAM role
authentication. This requires a corresponding change to goamz.

/cc @markpeek I think this is much cleaner. Also, I made it so it
doesn't append by default... I like the idea of appending but it scares
me that for a bug report someone might put like a multi-megabyte gist of
logs at me. HAHA. We'll see.