overlay.d: only run coreos-enable-network on first boot

I noticed recently that Azure is bringing up networking in the initramfs
on every boot [1]. This behavior was originally introduced in c1cf903. After
some discussion with @jlebon and @bgilbert we are going to convert the
coreos-enable-network.service to only be pulled in and enalbed on the
Ignition boot so that it doesn't run on subsequent boots.

[1] coreos/fedora-coreos-tracker#1383 (comment)

overlay.d/05core/usr/lib/dracut/modules.d/35coreos-network/module-setup.sh

@@ -11,7 +11,7 @@ install() {
     inst_simple "$moddir/coreos-enable-network.sh" \
         "/usr/sbin/coreos-enable-network"
     install_and_enable_unit "coreos-enable-network.service" \
-        "initrd.target"
+        "ignition-complete.target"
 
     inst_simple "$moddir/coreos-copy-firstboot-network.sh" \
         "/usr/sbin/coreos-copy-firstboot-network"