gitian: Debian 9 doesn't ship with ifconfig

[Sjors]

The instructions here won't work if ifconfig is not present:

https://github.com/bitcoin-core/docs/blob/master/gitian-building/gitian-building-setup-gitian-debian.md#setting-up-debian-for-gitian-building

Solution is to tell user to sudo apt-get install net-tools or change the instructions to use the ip command.

That still doesn't work for me. I had to sudo -s and then source /etc/rc.local.

(also, there's some inconsistency between this user being called debian vs gitianuser, but that creates a clear error message. However gitian-builder/var/lxc.config assumes debian)

[Sjors]

also, there's some inconsistency between this user being called debian vs gitianuser

See #20

[Sjors]

This is a Debian 9 issue. It works fine in Debian 8.

Probably still worth addressing at some point. Can we get a "Debian 9" label? cc @fivepiece @laanwj @MarcoFalke.

[fivepiece]

We could use

brctl addbr br0
ip addr add 10.0.3.2/24 brd 10.0.3.255 dev br0
ip link set br0 up

to set up the bridge instead of ifconfig. I haven't tested it on a NAT'd host VM though.

[maflcko]

See #22 for something that seems to work for me.

[Sjors]

Fixed through #22.