replace yunohost app setting by helper

scripts/install

@@ -150,8 +150,8 @@ if [[ ! -v ip6_net ]]; then # if ip6_net not set
   ip6_addr=none
 
   if [[ -e /tmp/.ynh-vpnclient-started ]]; then
-    vpnclient_ip6_net=$(sudo yunohost app setting vpnclient ip6_net 2>&1)
-    vpnclient_ip6_addr=$(sudo yunohost app setting vpnclient ip6_addr 2>&1)
+    vpnclient_ip6_net=$(sudo ynh_app_setting_set vpnclient ip6_net 2>&1)
+    vpnclient_ip6_addr=$(sudo ynh_app_setting_set vpnclient ip6_addr 2>&1)
 
     if [[ $vpnclient_ip6_net =~ :: && $vpnclient_ip6_addr =~ :: ]]; then
       ip6_net=${vpnclient_ip6_net}