As of now SSL certificates used by SSTP VPN are only allowed to be stored in `/config/user-data/sstp` this is in contrairy to othe SSL based VPN solutions (OpenVPN) where the key can be stored anywhere on the system. If the user stores the key outside of `/config` a warning is shown indicating that the key will be lost on upgrade.
Switching from a relative path in `/config/user-data/sstp` to an absolut path also gives the advantage of using certbot for SSTP VPN as the cert can be directly referenced.