Disable Forums for Docker devstack

This service is not currently available in Docker devstack.

lms/envs/devstack_docker.py

@@ -30,7 +30,8 @@ FEATURES.update({
     'ENABLE_COURSEWARE_SEARCH': False,
     'ENABLE_COURSE_DISCOVERY': False,
     'ENABLE_DASHBOARD_SEARCH': False,
-    'SHOW_LANGUAGE_SELECTOR': True
+    'SHOW_LANGUAGE_SELECTOR': True,
+    'ENABLE_DISCUSSION_SERVICE': False,
 })
 
 ENABLE_MKTG_SITE = os.environ.get('ENABLE_MARKETING_SITE', False)