Add unmanaged_files to the autoyast export
Add SystemDescription#description_path convenience method
Copy system description content into the autoyast export directory

The export directory needs to be made available to autoyast via network
during installation so that the extracted files can be put into the
system.
Add unmanaged files filter to AutoYaST export
Be more thorough when importing config_files and changed_managed_files

The script now doesn't try to download directories anymore (which fails)
and only sets the file attributes when they are available from the
manifest.json.
Use "http://" as the default entry for the system description URL
Adapt autoyast export integration test