units: order nspawn containers after network.target

This way we know that any bridges and other user-created network devices
are in place, and can be properly added to the container.

In the long run this should be dropped, and replaced by direct calls
inside nspawn that cause the devices to be created when necessary.

units/systemd-nspawn@.service.in

@@ -10,6 +10,7 @@ Description=Container %I
 Documentation=man:systemd-nspawn(1)
 PartOf=machines.target
 Before=machines.target
+After=network.target
 
 [Service]
 ExecStart=@bindir@/systemd-nspawn --quiet --keep-unit --boot --link-journal=try-guest --network-veth --machine=%I