This change maintains backwards compatibility, but makes the `accessKeyID` and `secretAccessKeySecretRef` fields of the `route53` DNS provider optional.
If not provided, AWS credentials will be loaded from `AWS_*` environment variables or the EC2 metadata service.
This should also work for things that impersonate the EC2 metadata service, such as [kube2iam](https://github.com/jtblin/kube2iam) and [kail](https://github.com/uswitch/kiam).
Signed-off-by: Matt Moyer <moyer@heptio.com>
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