Implement nova 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: I740be64fd5d822a4c4718e9b56b9fff34c2ba9da
Related-Prod: PROD-22099
diff --git a/README.rst b/README.rst
index 42ba8db..673df92 100644
--- a/README.rst
+++ b/README.rst
@@ -356,7 +356,7 @@
         ...
         networking: contrail
 
-Nova services on compute node with memcached caching:
+Nova services on compute node with memcached caching and security strategy:
 
 .. code-block:: yaml
 
@@ -371,6 +371,10 @@
             port: 11211
           - host: 127.0.0.1
             port: 11211
+          security:
+            enabled: true
+            strategy: ENCRYPT
+            secret_key: secret
 
 Client-side RabbitMQ HA setup: