Auth related env variables for customizing cassandra.yaml

[dfinlay]

Add support for customizing the authenticator, authorizer, role_manager values in cassandra.yaml via environmentals. This does not solve all of the pain points with initial auth setup, but it helps.

May be a start to addressing #106.

[tetafro]

Please merge this. How can anyone use the image, that gives no way to setup authentication process?

[dfinlay]

@tetafro I think the obvious answer is the image is even easier to use when there isn't any authentication :).

In less pithy news, we've been using my fork in Production since early August. Hundreds of ephemeral environments up and down without issue. It's not the ideal way to do it, so if anyone has a better suggestion, please throw it out there.

I was thinking about some sort of yaml merging facility, overlaying a provided yaml document on top of the defaults.

[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.