Keystone should use memcached for tokens

Right now some projects install memcached (such as Swift) but Keystone is not set up to use memcached.

When Keystone does not have a token cache method (such as memcached) configured, keystonemiddleware will throw the following error:
2016-03-11 02:48:21.508 17343 WARNING keystonemiddleware.auth_token [-] Using the in-process token cache is deprecated as of the 4.2.0 release and may be removed in the 5.0.0 release or the 'O' development cycle. The in-process cache causes inconsistent results and high memory usage. When the feature is removed the auth_token middleware will not cache tokens by default which may result in performance issues. It is recommended to use memcache for the auth_token token cache by setting the memcached_servers option.