openvpn: T56: remove strict checks for tls cert-file and key-file
openvpn: T56: remove strict checks for tls cert-file and key-file

This makes the tls cert-file and key-file optional and allows for more
advanced configurations via "openvpn-option", such as pkcs11 or pkcs12
options.

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x