Add MONGO_DB_CONNECTION_STRING to override the connection string bu…

…ilder

This avoids the need of having to specify the following environment variables:
`MONGO_DB_HOST`, `MONGO_DB_PORT`, `MONGO_DB_USER`, `MONGO_DB_PASSWORD` and `MONGO_DB_AUTHSOURCE` and allows greater control of the connection string.

app-init.js

@@ -40,6 +40,11 @@ const sessionStore = new MySQLStore({
 
 
 function getMongoDbConnectionString () {
+  // Allow the connection string builder to be overridden by an environment variable
+  if (process.env.MONGO_DB_CONNECTION_STRING) {
+    return process.env.MONGO_DB_CONNECTION_STRING;
+  }
+
   const host = process.env.MONGO_DB_HOST || 'localhost';
   const port = process.env.MONGO_DB_PORT || 27017;
   const user = process.env.MONGO_DB_USER || '';