mongoid config: add "parameterized" settings

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.
Merge pull request #54 from edx/sef/parameterized_mongoid_settings_release

edx-west/release -- add "environment" settings, a more flexible "prod"