fix(db_ops): rename db to add user on

Fixes NODE-1874

Author: kvwalker

lib/operations/db_ops.js

@@ -97,10 +97,10 @@ function addUser(db, username, password, options, callback) {
       const userPassword = md5.digest('hex');
 
       // If we have another db set
-      const db = options.dbName ? new Db(options.dbName, db.s.topology, db.s.options) : db;
+      const dbToUse = options.dbName ? new Db(options.dbName, db.s.topology, db.s.options) : db;
 
       // Fetch a user collection
-      const collection = db.collection(CONSTANTS.SYSTEM_USER_COLLECTION);
+      const collection = dbToUse.collection(CONSTANTS.SYSTEM_USER_COLLECTION);
 
       // Check if we are inserting the first user
       count(collection, {}, finalOptions, (err, count) => {