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
9 files changed