Make totpcgi-handler.pl a little more flexible

* We've been getting several folks asking questions about using
  FreeRADIUS against Active Directory. As the RADIUS handler wasn't
  tested against it it wasn't being flexible enough for the difference
  in userAttribute needed.

  As such, the attribute specifing the username in the directory is now
  configurable in the yaml by modifying userAttribute.

* Fix the example yaml to have userSearchBase instead of searchbase
  (since this was wrong)

Signed-off-by: Andrew Grimberg <agrimberg@linuxfoundation.org>