v3 auth API untranslatable to XML

The JSON spec for v3 authentication contains multiple root elements when translated to XML, which is not allowed.

This affects three things:

- Authentication requests have two root elements: "authentication" and "scope"
- Authentication responses have many root elements, including "user", "expires", "scope", etc
- Authentication-related errors have two root elements: "error" and "authentication"

This needs to be fixed at the API level (openstack/identity-api) along with the current server implementation. Client implementation is currently WIP.

Reported by gyee.