use unshare to create new user-root namesapce to have the perrmission to
create network device, use mount to update files information of network
device in /sys(nemu)
test use iptables to manage route condition, miniupnpd to manage upnp,
brctl(nemu) to create bridge, so add them in component.

This version of scons only support python2

See merge request !1140

This partially reverts ee733f9d because
of issues with python3

Commit ee733f9d (version up: zc.buildout 2.7.1+slapos017,
slapos.libnetworkcache 0.23, 2022-03-28) introduced a new way of
computing cache key used for shacache and downlaod URLs that were only
in cache but no longer available from their original location no longer
could be retrieved. This update the original download URLs to URLs that
are valid today.

See merge request nexedi/slapos!1147

I could not find tarball for 2.1 so I updated

also remove useless environment option which has same value as the macro

(+ small cosmetic fixes)

All instances in the wild have now moved their DB
from srv/res6stnet/registry to srv/re6stnet.

See commit a1ed9c30.