Add confirmation to vpn user script when overwriting config

[Bill Monkman]

If you run the vpn setup script multiple times for the same environment it will overwrite your wireguard config file. Add a confirmation prompt if the file already exists. If they don't want to overwrite, maybe just print it?

https://github.com/commitdev/zero-aws-eks-stack/blob/main/templates/scripts/add-vpn-user.sh