Implements memcache storage for keystone tokens

Switching keystone token storage to memcache with dogpile cache implementation.
This allows to use memcache as non persistence storage in HA mode.

Change-Id: I511f5849206455d65ca11ecbb8e8c1c3f639fbcb
Closes-Bug: 1269819
Merge "Implements memcache storage for keystone tokens"