slapos.cfg.example: clean up [networkcache]

When using HTTPS, slapos.libnetworkcache uses exclusively the client certificate
(which must be configured) so https://*.nxdcdn.com can't work from slapos.libnetworkcache

And without download-dir-url, download-cache-url is quite useless.

Reorder options to clarify the difference between upload & https.