Optimizations (dot cube files, cron and OpenVPN options)

[jvaubourg]

1. We should use dot cube files for configuring VPN Client (new feature).
2. In neutrinet.sh: Additional OpenVPN option ns-cert-type server was integrated in the default configuration, and resolv-retry infinite is useless because infinite is the default value.
3. There is now a systemd restart on-failure service, overriding the default service of OpenVPN. So, the cron is useless.