use yunohost helper instead of cli

conf/openvpn_90-hotspot

@@ -18,7 +18,7 @@ set_nat() {
 }
 
 if systemctl is-active __SERVICE_NAME__; then
-  old_gateway_interface=$(yunohost app setting --app=__APP__ --key=gateway_interface)
+  old_gateway_interface=$(ynh_app_setting_get --app=__APP__ --key=gateway_interface)
   new_gateway_interface=$(ip route get 1.2.3.4 | awk '{ print $5; }')
 
   if [[ -n "$old_gateway_interface" ]] && [[ "$old_gateway_interface" != "$new_gateway_interface" ]] && is_nat_set "$old_gateway_interface"; then