Add support for using an authentication database for MongoDB.

A popular convention is to have user accounts stored in a separate authentication
database. This change add support for configuring edx-platform to work with
such a setup.

common/djangoapps/track/backends/mongodb.py

@@ -29,6 +29,7 @@ class MongoBackend(BaseBackend):
           - `password`: collection user password
           - `database`: name of the database
           - `collection`: name of the collection
+          - 'auth_source': name of the authentication database
           - `extra`: parameters to pymongo.MongoClient not listed above
 
         """
@@ -46,6 +47,8 @@ class MongoBackend(BaseBackend):
         db_name = kwargs.get('database', 'track')
         collection_name = kwargs.get('collection', 'events')
 
+        auth_source = kwargs.get('auth_source') or None
+
         # Other mongo connection arguments
         extra = kwargs.get('extra', {})
 
@@ -67,7 +70,7 @@ class MongoBackend(BaseBackend):
         database = self.connection[db_name]
 
         if user or password:
-            database.authenticate(user, password)
+            database.authenticate(user, password, source=auth_source)
 
         self.collection = database[collection_name]