lxc, lxd: If we're running systemd, wait until the network is up

We execute `apt-get update' more or less as soon as the container is
started. In some situations this is too early: it can be before network
is fully working.

If we have systemd, use network-online.target to wait until it thinks
networking is up.

Modified-By: Martin Pitt <mpitt@debian.org>
  - Add the same approach to LXC
  - Some simplification

LP: #1749736