An unifed caching system for middlewares

With the included middle-ware we currently have auth_token doing its own caching via memcache while ec2 and s3 middlewares doesn't cache validated token received from keystone.

We could use common functions but those middle-ware are supposed to be dropped as in and it would not be nice to have another dependence.

We could have a cache middleware like used in swift ;

https://github.com/openstack/swift/blob/master/swift/common/middleware/memcache.py

which we could reuse in keystone for cache management.