Changes PlatformManager to support sudo -u option.

* get_current_user now returns the name of the user sudo is
  running the command as (the effective user) when using the
  sudo -u option to run as another non-root user.
* get_current_user now returns 'root' when the real user
  is 'root'.

Change-Id: I18a218f3401a73854e9d8888c553a9c7a48ba084