AWS client tools commonly support passing credentials via
`AWS_ACCESS_KEY_ID` + `AWS_SECRET_ACCESS_KEY`, but supporting only this is
insufficient. For example, access key IDs provided by STS require passing in
`AWS_SECURITY_TOKEN` as a third value, and EC2 instances are often provided
dynamic credentials at runtime via the EC2 metadata service.
This changeset makes `lego` attempt to find credentials in the same way that
the `aws` CLI tool attempts to find credentials. The result is even less
auth code than before because `goamz` provides all this with `aws.GetAuth()`.
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