make s3_bucket optional, add cache.db option and properly inherit common...

... in config.yml

A recent change in docker-registry made "standalone" default to false and the config.yml template prod section didn't inherit the common section properly.

I've also made s3_region optional as boto got confused with us-east-1 (US Standard) and worked alright defaulting to it, plus added boto_bucket = s3_bucket, as that seemed necessary. There's a couple of open issues for that on docker-registry iirc.