add "environment" settings, a more flexible "prod"

Instead of hardcoding database names and such into these config files, get them from the environment.  This should be more flexible.  This is what we're using for our production comment service at Stanford.

This is a bit of a hack, it seems like the right thing to do would be to parameterize the "production" setting itself so that edX.org database names weren't hardcoded in here, but without knowing the code/ops to do so I was unwilling to do that.  So just adding my own section.

@e0d, @kevinchugh or @jimabramson, can you code review please?