Add Cached loader for templates

This patch adds the cached template loader to our TEMPLATE_LOADERS
setting.

Change-Id: I44bb13dfb2868c857112483c1ad09a181121b58c
Implements: blueprint cache-templates

openstack_dashboard/settings.py

@@ -127,9 +127,11 @@ TEMPLATE_CONTEXT_PROCESSORS = (
 )
 
 TEMPLATE_LOADERS = (
-    'django.template.loaders.filesystem.Loader',
-    'django.template.loaders.app_directories.Loader',
-    'horizon.loaders.TemplateLoader',
+    ('django.template.loaders.cached.Loader', (
+        'django.template.loaders.filesystem.Loader',
+        'django.template.loaders.app_directories.Loader',
+        'horizon.loaders.TemplateLoader',
+    )),
 )
 
 TEMPLATE_DIRS = (

releasenotes/notes/bp-cache-templates-4ab00dcda195a03a.yaml

@@ -0,0 +1,8 @@
+---
+features:
+  - Added the Django template cached loader, so templates are stored in memory.
+    See https://docs.djangoproject.com/en/1.8/ref/templates/api/#django.template.loaders.cached.Loader
+upgrade:
+  - Custom template tags must have a thread-safe Node implementation to work
+    with the the cached loader. See
+    https://docs.djangoproject.com/en/1.8/howto/custom-template-tags/#template-tag-thread-safety