carnegie-app: adding ability to migrate back in

They have their own LMS DB.

playbooks/edx-west/carnegie-prod-app.yml

@@ -3,6 +3,11 @@
 
 - hosts: ~tag_Name_app(10|20)_carn
   sudo: True
+  vars_prompt:
+    - name: "migrate_db"
+      prompt: "Should this playbook run database migrations? (Type 'yes' to run, anything else to skip migrations)"
+      default: "no"
+      private: no
   vars:
     secure_dir: '../../../edx-secret/ansible'
     # this indicates the path to site-specific (with precedence)