Drop client_type for auth module

Client type was designed to allow the auth module to consume
the official keystone client for tests based on the official clients.
Scenario tests have been migrated to tempest client, and CLI tests
are being moved out of tempest, and they don't need an external
auth provider anyways.
There's no need for client_type anymore, removing it.

Change-Id: I6eebd80ea2259b72e8013e65e9a83615f86a3f84
Partially-implemnts: bp/tempest-client-scenarios
Merge "Drop client_type for auth module"