Support storing cert-info in S3

[mlc]

For some use cases, it may be inconvenient to redeploy the lambda every time the certificate-info changes. This allows the option of storing the certificate info in a JSON file in the s3-account-bucket, while still supporting storing the info directly in the config for cases where that is simpler.

[larryboymi]

Hey Mike.... thanks for contributing! I'm planning to go back through the codebase in the next couple of weeks and I'll definitely implement a suggestion like this (although final details might differ slightly). Thanks again!