Discover Service endpoints via a plugin

Most clients don't actually care what URL they use to communicate with a server. They just care that the URL belongs to the service type they expect.

This information linking the service type to a URL comes from the service catalog which is received as part of a user's token. Because user tokens are now managed by auth plugins it makes sense that the service catalog and thus the endpoint determination should be managed by the auth plugin as well.

So rather than specifying the full URL that a request should be made from it should be able to just say:

session.request('/users', service_type='identity', endpoint_type='admin')

and have the plugin/session figure the rest out for it.

This will give us a common place to plug endpoint version discovery into in the future.