Implement panko 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: I70758868fb3cd1f78a4b5410baec2d509ebca105
Related-Prod: PROD-22099
diff --git a/tests/pillar/server_cluster.sls b/tests/pillar/server_cluster.sls
index 79e7732..de177d1 100644
--- a/tests/pillar/server_cluster.sls
+++ b/tests/pillar/server_cluster.sls
@@ -31,6 +31,10 @@
           port: 11211
         - host: 127.0.0.1
           port: 11211
+      security:
+        enabled: true
+        strategy: ENCRYPT
+        secret_key: secret
     logging:
       log_appender: false
       log_handlers: