Export AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY

I've got some other ideas in mind, but they might raise some discussion, so here's a quickie I'd like to get out of the way.

This naming scheme is used by the new official [aws-cli client][1], so hopefully it is becoming somewhat of a standard. It stems from the underlying boto Python library, so incidentally it automatically serves other tools like Ansible too.

[1]: https://github.com/aws/aws-cli