Change AuthPrivateKey decorator to Authentication decorator

[himdel]

Authentication is the base class, AuthPrivateKey a child.
Previously (ManageIQ/manageiq#18633) CloudManager::AuthKeyPair inherited from AuthPrivateKey, but changed to Authentication now.

So, moving the decorator to cover the whole base class.
(Not seeing any problem with AuthToken, AuthUseridPassword, and ManageIQ::Providers::AutomationManager::Authentication also gaining a decorator because of this.)

(Fixes failing travis on ui-classic master:

  1) AuthKeyPairCloudController#report_data when tile mode is selected returns key pairs with quadicons
     Failure/Error: expect(results['data']['rows'][0]['quad']).to have_key('fonticon')
       expected  to respond to `has_key?`
     # ./spec/controllers/auth_key_pair_cloud_controller_spec.rb:98

)

Cc @skateman , @agrare

[himdel]

(Verified ui-classic spec and rake in this repo is passing locally for me with this change.)