[keyserver] Introduce getDBConfig that can source from environmental variables

Summary:

• As we Dockerize the keyserver, we need to make it possible to define config via environmental variables.
• In most of those cases, we can define the entire JSON file as an environmental variable.
• But for the case of the DB config, we'd like to be able to share a config style with the mysql Docker image, which uses separate environmental variables for each key
• So we're going to special-case DB config here
• The rest of the configs will be handled through importJSON in a following diff

Depends on D4025

Test Plan:

1. Test to make sure the keyserver still runs and can load the website with db_config.json defined
2. Test to make sure scripts that need DB access (such as reset-password.js) still runs with db_config.json defined
3. In combination with a later diff I'll make sure to test that the environmental variables work too

Reviewers: palys-swm, atul

Reviewed By: palys-swm, atul

Subscribers: Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4097