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: Would it work to simply make this:

```python
RackspaceAuthentication.patch_management_url()
return super(self.__class__, self).get_client()
```