List endpoints for an object, account or container

This blueprint implements the ability to list endpoints for an object, account or container, to make it possible to integrate swift with software that relies on data locality information to avoid network overhead.

In particular, it was implemented for usage with Hadoop - see https://issues.apache.org/jira/browse/HADOOP-8545

A piece of middleware is added which answers requests of the form: "/endpoints/{account}/{container}/{object}" with a list of endpoints of the form: "http://{server}:{port}/{device}/{partition}/{account}/{container}/{object}" and similarly for accounts and containers.

The service is intended for consumption by third-party services living inside the cluster (as the endpoints make sense only inside the cluster behind the firewall); potentially written in a different language. This is why it's provided as a REST API and not just a Python API: to avoid requiring clients to write their own ring parsers in their languages, and to avoid the necessity to distribute the ring file to clients and keep it up-to-date.