Implement Ironic 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: Ie252472e7e000ef76660c731b293a25dc21aa433
Related-Prod: PROD-22099
diff --git a/README.rst b/README.rst
index a0bb932..72acd55 100644
--- a/README.rst
+++ b/README.rst
@@ -173,3 +173,23 @@
 
 You can read more about it here:
     https://docs.openstack.org/security-guide/messaging/security.html
+
+Ironic service with cache and security enabled
+
+.. code-block:: yaml
+
+  ironic:
+    api:
+      enabled: true
+      version: pike
+        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