Added ENV option for authentication and authorization

[eintopf]

I added options for authentication / authorization (via CASSANDRA_* ENV).

[eintopf]

I don't get why CI is failing? What is the issue?

[tianon]

I think this is probably going to lead down a similar path as the discussion in #39 (comment) -- namely, a lot of authenticators are likely to require even more configuration, so it's probably easier for users who need a custom authenticator to create their own configuration file with the necessary bits, as described in the comment thread I've linked.

[tianon]

I'm sorry, but honestly if we're going to do anything more with environment variables it would likely be in the opposite direction (which we won't do due to the backwards compatibility implications) -- modifying a YAML file via shell script is very fragile, and setting complex configuration in this way is not great. For custom configuration, the only/best recommendation is to provide an appropriate cassandra.yaml file to your running instance (via a new image with FROM / COPY in a short Dockerfile, via Docker Configs, via bind mount, etc).

Closing in favor of #160 + docker-library/docs#1317.