WIP: introduce strongswan

hosts/elrond/default.nix

hosts/elrond/links.nix

hosts/elrond/strongswan.nix

hosts/elros/default.nix

hosts/elros/strongswan.nix

hosts/varda/default.nix

hosts/varda/strongswan.nix

lib/kloenk-vpn.cert.pem

lib/vpn/elrond.cert.pem

lib/vpn/elros.cert.pem

lib/vpn/varda.cert.pem

profiles/strongswan.nix

secrets/elrond.yaml

secrets/elros.yaml

secrets/varda.yaml