Refactor networking service into systemd-networkd

[kaihendry]

Right now we are using https://github.com/Webconverger/webc/blob/master/etc/network/interfaces style way of getting online. And this can be really slow, esp on Virtualbox!

    1min 18.277s networking.service

The log http://ix.io/kub does not immediately suggest what's going wrong, but the dhcp client seems to be overworking.

[kaihendry]

http://s.natalian.org/2016-01-25/revamp-networking.png

In 99becac there was a systemd-networkd update (215-17+deb8u3), which I think makes it safe to start this work.