Enable specification of modulestore in LMS aws config

lms/envs/aws.py

@@ -76,6 +76,10 @@ DATABASES = AUTH_TOKENS['DATABASES']
 
 XQUEUE_INTERFACE = AUTH_TOKENS['XQUEUE_INTERFACE']
 
+# Get the MODULESTORE from auth.json, but if it doesn't exist,
+# use the one from common.py
+MODULESTORE = AUTH_TOKENS.get('MODULESTORE', MODULESTORE)
+
 if 'COURSE_ID' in ENV_TOKENS:
     ASKBOT_URL = "courses/{0}/discussions/".format(ENV_TOKENS['COURSE_ID'])