Configure variables in `env`

Add route on host, enable and allow forwarding:
```
ip route add ${openvpn_subnet} via 10.116.0.10 src ${eth0_ip}
sysctl -w net.ipv4.ip_forward=1
iptables -A FORWARD -j ACCEPT
```

where `openvpn_subnet` is the subnet configured in `env` (OVPN_SUBNET_ADDR and OVPN_SUBNET_MASK) in CIDR form (x.x.x.x/xx) and `eth0_ip` is the visible ip of the host on the local (physical, not VPN) network.

Do not forget to add a route on your default gateway