Implement neutron 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: I7f623586023c8b0605bb77977a28d6b7c47e5287
Related-Prod: PROD-22099
diff --git a/README.rst b/README.rst
index f5169b5..187a911 100644
--- a/README.rst
+++ b/README.rst
@@ -1473,6 +1473,25 @@
           eventletwsgi:
             level: 'DEBUG'
     ......
+Neutron server with memcached caching and security strategy:
+
+.. code-block:: yaml
+
+    neutron:
+      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
 ========