adapt gateway selection

depends on leap/container-platform/glue#12

• adapt eip-service.json parsing, if we agree on option 2 in leap/container-platform/glue#12 - ip_address needs to be interpreted as array

• prefer ipv6 over ipv4: try to connect to ipv6 first, fall back to ipv4 if the former fails

• it seems openvpn 2.4 is able to handle a dual-stack setup, so maybe there's not much to do for checkbox 2. See https://wiki.archlinux.org/index.php/OpenVPN#Connect_to_the_server_via_IPv6, https://serverfault.com/questions/651832/openvpn-with-mixed-ipv4-and-ipv6-clients