if no config changed, don't forget the old config

Change-Id: I24509dce1a218e67d8438c5405506dcdc711cd33
Reviewed-on: https://gerrit.instructure.com/126702


Reviewed-by: Rob Orton <rob@instructure.com>
Tested-by: Jenkins
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

config/initializers/cache_store.rb

@@ -44,6 +44,8 @@ load_cache_config = -> do
 
     if last_cluster_cache_config != config
       cache_map[cluster] = Canvas.lookup_cache_store(config, cluster)
+    else
+      cache_map[cluster] = Switchman.config[:cache_map][cluster]
     end
   end