Fix manager->get_auth_provider interface

Tempest is defining stable interface of service clients
handling in lib but need to maintain the old interface
tempest.manager.get_auth_provider() as it is used by many
plugins and provide them a grace period to move to
stable interface.

This interface got broken while doing service clients work.
This patch fixing the same and fix gate for plugin using
this unstable interface.

Change-Id: I5733f20f5fba3c1aa1318405f5af976513eebf14
1 file changed