also fix un-variablized lms-preview upstart script

playbooks/roles/gunicorn/templates/lms-preview.conf.j2

@@ -18,10 +18,10 @@ env LANG=en_US.UTF-8
 env DJANGO_SETTINGS_MODULE=lms.envs.aws
 env SERVICE_VARIANT="lms-preview"
   
-chdir ${app_base_dir}/mitx
+chdir {{platform_code_dir}}
 setuid www-data
 
-exec ${venv_dir}/bin/gunicorn --preload -b 127.0.0.1:$PORT -w $WORKERS --timeout=300 --pythonpath=${app_base_dir}/mitx lms.wsgi
+exec ${venv_dir}/bin/gunicorn --preload -b 127.0.0.1:$PORT -w $WORKERS --timeout=300 --pythonpath={{platform_code_dir}} lms.wsgi
 
 post-start script
   while true