WIP: Add dns challenge support

[gianrubio]

POC for supporting dns challenge
Write a provider for route53

This is WIP, please don't merge!
Fix #26

[gianrubio]

I didn't add aws vendor repo to make easy to compare the changes, to run this POC run dep ensure --update

[jetstack-bot]

@gianrubio: PR needs rebase.

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[totemcaf]

Hi @gianrubio, I'm very interested in this feature. Is there any news on it, can I help?

[munnerz]

Given the status of kube-lego (EoL), we won't be adding DNS support to this project.

Instead, please have a look at cert-manager. It supports ACME DNS validation as well as HTTP, as well as a number of other features.

Thanks for putting the work together initially @gianrubio, and sorry this PR wasn't looked at. I'm going to close this now as we won't be accepting new features into kube-lego.