Provide option for auth_token middleware to do memcache encryption & integrity check

As we know, memcache servers are not very well protected, which
means anyone have access to them can replace information stored there. Therefore, it would be beneficial for Keystone auth_token middleware to provide an option to either encrypt or hmac the values stored in the memcache.

This feature shall accept two configurable options:

1) memcache_value_treatment - ENCRYPT, MAC, or default to unprotect.
2) encryption_key_derivation_secret - key derivation secret for generating
the encryption or hmac key

Granted, PKI tokens makes memcache less relevant. But not everyone is ready
to switch over to PKI tokens just yet. So in the short term, memcache
protection is still very useful.