Add credentials service username to common cms

We're now calling programs-related code from CMS so we need to have the
username for the credentials service available in CMS

cms/envs/common.py

@@ -2133,6 +2133,7 @@ ECOMMERCE_API_SIGNING_KEY = 'SET-ME-PLEASE'
 
 CREDENTIALS_INTERNAL_SERVICE_URL = 'http://localhost:8005'
 CREDENTIALS_PUBLIC_SERVICE_URL = 'http://localhost:8005'
+CREDENTIALS_SERVICE_USERNAME = 'credentials_service_user'
 
 ANALYTICS_DASHBOARD_URL = 'http://localhost:18110/courses'
 ANALYTICS_DASHBOARD_NAME = 'Your Platform Name Here Insights'