Add service username in cms settings

cms/envs/common.py

@@ -60,6 +60,10 @@ from lms.envs.common import (
 
     # Django REST framework configuration
     REST_FRAMEWORK,
+
+    # the following setting is referenced in a migration in openedx that gets
+    # run under both lms and cms, so it must be available in both places.
+    CREDENTIALS_SERVICE_USERNAME
 )
 from path import Path as path
 from warnings import simplefilter