nuageinit: implement write_files

write_files is a list of files that should be created at the first boot

each file content can be either plain text or encoded in base64 (note
that cloudinit specify that gzip is supported, but we do not support it
yet.)

All other specifier from cloudinit should work:
by default all files will juste overwrite exesiting files except if
"append" is set to true, permissions, ownership can be specified.
The files are create before packages are being installed and user
created.

if "defer" is set to true then the file is being created after packages
installation and package manupulation.

This feature is requested for KDE's CI.