Strongswan after network-online instead of network

The systemd service file shipped with strongswan has strongswan started after `network-online`. It turns out that this is for good reason: failure to connect on boot otherwise. 

See this thread on the mailing list, which my colleague initiated after finding that our NixOS strongswan config wouldn't connect on boot:
https://lists.strongswan.org/pipermail/users/2017-January/010359.html

Tested on a local config (which has the strongswan service config overridden).