refactor(config.js): Extract config file

* Separate different config source to each files
* Freeze config object

lib/config/defaultSSL.js

@@ -0,0 +1,17 @@
+'use strict'
+
+const fs = require('fs')
+
+function getFile (path) {
+  if (fs.existsSync(path)) {
+    return path
+  }
+  return undefined
+}
+
+module.exports = {
+  sslkeypath: getFile('/run/secrets/key.pem'),
+  sslcertpath: getFile('/run/secrets/cert.pem'),
+  sslcapath: getFile('/run/secrets/ca.pem'),
+  dhparampath: getFile('/run/secrets/dhparam.pem')
+}