enable networkd for live-server systems

the logic for doing this is the same as why it is done for cloud-images, for which
see http://bazaar.launchpad.net/~ubuntu-core-dev/livecd-rootfs/trunk/revision/1520

I think this behaviour is a bug in cloud-init (see lp:#1737630) but this is a good
workaround in the mean time

debian/changelog

@@ -1,3 +1,9 @@
+livecd-rootfs (2.488) UNRELEASED; urgency=medium
+
+  * Enable systemd-networkd by default for live-server installs.
+
+ -- Michael Hudson-Doyle <mwhudson@debian.org>  Thu, 21 Dec 2017 10:59:50 +1300
+
 livecd-rootfs (2.487) bionic; urgency=medium
 
   * Drop android-tools-fsutils dependency, and instead install it on

live-build/ubuntu-server/includes.chroot/etc/systemd/system/multi-user.target.wants/systemd-networkd.service

@@ -0,0 +1 @@
+/lib/systemd/system/systemd-networkd.service

live-build/ubuntu-server/includes.chroot/etc/systemd/system/network-online.target.wants/systemd-networkd-wait-online.service

@@ -0,0 +1 @@
+/lib/systemd/system/systemd-networkd-wait-online.service

live-build/ubuntu-server/includes.chroot/etc/systemd/system/sockets.target.wants/systemd-networkd.socket

@@ -0,0 +1 @@
+/lib/systemd/system/systemd-networkd.socket