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.
25 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Master

01:55:18

passed SlapOS.Eggs.UnitTest-Master.Python2

00:52:14

failed SlapOS.Eggs.UnitTest-Master.Python3

00:45:58

failed SlapOS-Master.UnitTest-Master

08:14:20

passed SlapOS.Eggs.UnitTest-Master.Python2

00:16:01

failed SlapOS.Eggs.UnitTest-Master.Python2

00:11:12

passed SlapOS.Eggs.UnitTest-Master.Python2

00:14:53

passed SlapOS.Eggs.UnitTest-Master.Python2

00:20:06

passed SlapOS.Eggs.UnitTest-Master.Python2

01:03:39

passed SlapOS.Eggs.UnitTest-Master.Python2

00:52:14

passed SlapOS.Eggs.UnitTest-Master.Python2

00:10:55

passed SlapOS.Eggs.UnitTest-Master.Python2

00:21:50

passed SlapOS.Eggs.UnitTest-Master.Python2

00:21:23

failed SlapOS.Eggs.UnitTest-Master.Python2

00:44:34

passed SlapOS.Eggs.UnitTest-Master.Python2

00:17:44

passed SlapOS.Eggs.UnitTest-Master.Python3

00:37:47

passed SlapOS.Eggs.UnitTest-Master.Python3

00:21:07

passed SlapOS.Eggs.UnitTest-Master.Python3

00:16:05

passed SlapOS.Eggs.UnitTest-Master.Python3

failed SlapOS.Eggs.UnitTest-Master.Python3

01:03:25

failed SlapOS.Eggs.UnitTest-Master.Python3

00:25:58

passed SlapOS.Eggs.UnitTest-Master.Python3

00:19:57

passed SlapOS.Eggs.UnitTest-Master.Python3

00:24:45

passed SlapOS.Eggs.UnitTest-Master.Python3

00:22:23

failed SlapOS.Eggs.UnitTest-Master.Python3

01:25:03