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/lib/xmodule/xmodule/mongo_utils.py

@@ -36,6 +36,9 @@ def connect_to_mongodb(
         # No 'replicaSet' in kwargs - so no secondary reads.
         mongo_client_class = pymongo.MongoClient
 
+    # If the MongoDB server uses a separate authentication database that should be specified here
+    auth_source = kwargs.pop('auth_source', '') or None
+
     # If read_preference is given as a name of a valid ReadPreference.<NAME> constant
     # such as "SECONDARY_PREFERRED", convert it. Otherwise pass it through unchanged.
     if 'read_preference' in kwargs:
@@ -59,10 +62,9 @@ def connect_to_mongodb(
             mongo_conn,
             wait_time=retry_wait_time
         )
-
     # If credentials were provided, authenticate the user.
     if user is not None and password is not None:
-        mongo_conn.authenticate(user, password)
+        mongo_conn.authenticate(user, password, source=auth_source)
 
     return mongo_conn