As we Dockerize the keyserver, we need to make it possible to define config via environmental variables. This is a standard practice in Docker for specifying config/secrets.
Currently we specify config/secrets to Node via JSON files that get imported directly. In previous diffs in the stack, I've refactored all of these codepaths to go through importJSON. In this diff, I make it possible for importJSON to get the config from environmental variables rather than JSON files.
Depends on D4101