[NFC] Outline config creation into a separate module

This is done in order to have config creation rules
unified in one place to avoid modifying them multiple times
if they change.

We already had 3 duplicated pieces of code initializing the
config and there will be more config uses in future tests.

This resolves #1960

server/config.js

@@ -0,0 +1,5 @@
+module.exports = require('nconf')
+    .argv()
+    .env({ lowerCase: true })
+    .file('environment', { file: `config/${process.env.NODE_ENV}.json` })
+    .file('defaults', { file: 'config/default.json' });