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.
fix: simplify get_client from RackspaceAuthentication class

Note: The exceptions thrown will still say OpenstackAuthentication
in the message
refactor: rm ununsed keystone client
refactor: use datacenter context for generating management_url
Merge pull request #15 from TheSriram/patch_management_url_rackspace

feat: patch management url for Rackspace Authentication