Add Identity v2 role and service tests

* Add current auth info (auth_ref) to ClientManager
* Fix identity.v2_0.role.ListUserRole to get default user/project
  from ClientManager.auth_ref
* Fix identity.v2_0.role.AddRole call to roles.add_user_role()

Change-Id: Ie8bf41c491d97b0292a2b86bdc9b7580989a7f97