Support other DNS challenges

[wesley-dean]

Allow one to pass CHALLENGE to the container; have it default to --dns-route53

[wesley-dean]

It needs to change the challenge in two places:

1. entrypoint: https://github.com/kdaweb/certbotbot/blob/master/entrypoint.sh#L83
2. Dockerfile: https://github.com/kdaweb/certbotbot/blob/master/Dockerfile#L1