Remove usage of credentials_factory.AdminManager

Tempest is making credentials_factory as stable interface
and will be removing the AdminManager class which is wrapper
of creating client manager with admin cred.

admin manager can be instantiated by providing the admin cred
to clients.Manager.

This commit removes the usage of AdminManager.

Change-Id: I978e82dcdaa37fbfa71907527be02f03f759c824
2 files changed