feat: patch management url for Rackspace Authentication

The keystone client expects an entry for identity in service catalog,
this patch, monkey patches the service catalog's urlfor method which
returns the management url from service catalog.

This patch was necessary because rackspace identity, does not have the
management url field in the returned service catalog.
Q: I'm wondering if it would be possible to make this take advantage of the datacenter parameter provided to RackspaceAuthentication instance so that it could call get_identity_apihost() to get the correct, localized one; _management_url() presently just returns the "default" URL.

Might be able to use the Service Catalog's "region" value too....

Just thinking...