Move unmanaged_files filter message to export_task

Instead of showing the unmanaged_files filter list during kiwi_build it
is no shown every time a description with unmanaged_files is exported to
Kiwi or Autoyast.
Merge pull request #482 from SUSE/move_unmanaged_filter_message_to_export_task

Move unmanaged_files filter message to export_task