Clean up shell authentication

* Remove the auth option checks as the auth plugins will validate
  their own options
* Move the initialization of client_manager to the end of
  initialize_app() so it is always called.  Note that no attempts
  to actually authenticate occur until the first use of one of the
  client attributes in client_manager.  This leaves
  initialize_clientmanager() (formerly uathenticate_user()) empty
  so remove it.
* Remove interact() as the client_manager has already been created
  And there is nothing left.
* prepare_to_run_command() is reduced to trigger an authentication
  attempt for the best_effort auth commands, currently the only
  one is 'complete'.
* Add prompt_for_password() to ask the user to enter a password
  when necessary.  Passed to ClientManager in a new kward pw_func.

Bug: 1355838
Change-Id: I9fdec9144c4c84f65aed1cf91ce41fe1895089b2
Merge "Clean up shell authentication"