Ability to use EC2 instance role for route53 permissions

[mcnicholls]

At the moment the AWS Access Key and AWS Secret Key are required when using the route53 provider.

It appears that the lego library will use EC2 instance roles if no environment variables are provided with keys.
Currently this container is checking and erroring if keys aren't provided. Maybe a checkbox could be added to indicate it should use an EC2 instance role?

That way it can still error if the intention is to use keys and they haven't been provided.