Implement heat memcache security strategy

Provides an option to authenticate and optionally encrypt the token
data stored in the cache:
memcache_security_strategy = MAC/ENCRYPT
memcache_secret_key = secret_key

Change-Id: Ice0259e12009bddec2607d54219e1d4b639839c2
Related-Prod: PROD-22099
diff --git a/README.rst b/README.rst
index 46a8fc6..7df62e0 100644
--- a/README.rst
+++ b/README.rst
@@ -272,6 +272,25 @@
 You can read more about it here:
     https://docs.openstack.org/security-guide/databases/database-access-control.html
 
+Heat services with memcached caching and security strategy:
+
+.. code-block:: yaml
+
+    heat:
+      server:
+        enabled: true
+        ...
+        cache:
+          engine: memcached
+          members:
+          - host: 127.0.0.1
+            port: 11211
+          - host: 127.0.0.1
+            port: 11211
+          security:
+            enabled: true
+            strategy: ENCRYPT
+            secret_key: secret
 Upgrades
 ========