None doesn't work in a complex value.

Setting a default value and adding a changelog entry.

CHANGELOG.md

+- Role: edxapp
+  - A new var was added to make it easy ot invalidate the default
+    memcache store to make it easier to invalidate sessions. Updating
+    the edxapp env.json files will result in all users getting logged
+    out.  This is a one time penalty as long as the value of `EDXAPP_DEFAULT_CACHE_VERSION`
+    is not explicitly changed.
+
 - Role: nginx
   - New html templates for server errors added.
     Defaults for a ratelimiting static page and server error static page.

playbooks/roles/edxapp/defaults/main.yml

@@ -346,7 +346,7 @@ EDXAPP_CELERY_WORKERS:
     service_variant: lms
     concurrency: 2
 
-EDXAPP_DEFAULT_CACHE_VERSION: "None"
+EDXAPP_DEFAULT_CACHE_VERSION: "1"
 
 #-------- Everything below this line is internal to the role ------------