We need a default docstore config defined even when we override it.

playbooks/roles/edxapp/defaults/main.yml

@@ -599,6 +599,7 @@ edxapp_generic_auth_config:  &edxapp_generic_auth
   AWS_ACCESS_KEY_ID:  "{{ EDXAPP_AWS_ACCESS_KEY_ID }}"
   AWS_SECRET_ACCESS_KEY: "{{ EDXAPP_AWS_SECRET_ACCESS_KEY }}"
   SECRET_KEY:  "{{ EDXAPP_EDXAPP_SECRET_KEY }}"
+  DOC_STORE_CONFIG: "{{ edxapp_generic_doc_store_config }}"
   XQUEUE_INTERFACE:
     basic_auth: "{{ EDXAPP_XQUEUE_BASIC_AUTH }}"
     django_auth: "{{ EDXAPP_XQUEUE_DJANGO_AUTH }}"