Migrate network configuration to the new OpenWrt 21.02 configuration

Complementary issue to foris-controller#204 (closed)

Adjust it to be able to read both old and new config, but allow storing only new config syntax.

https://gitlab.nic.cz/turris/foris-controller/foris-controller-openvpn-module/-/blob/master/foris_controller_backends/openvpn/init.py#L217-221