Add v2 API to list endpoints middleware

The new API adds better support for storage policies and changes the
response from a list of backend urls to a dictionary with a key
"endpoints" that's a list of of the backend urls and a new key headers
that's a dictionary of headers to send along with the backend request.
In the v2 response format for object requests, there the headers key
includes "X-Backend-Storage-Policy-Index" which indicates the storage
policy index for the endpoints returned in the response.

Change-Id: I706a5b5be8002c633fe97b2429256735800a902e
Merge "Add v2 API to list endpoints middleware"