Read the VPN server IP address from the monitoring node config

morph offers an auto-passed nodes parameter from which we can read all of the nodes in the morph "network". We can dig around in this to find the monitoring node and then read its configured IP address.

It may be worth noting that this address appears in at least two places in the configuration. It appears in our "input" configuration which grid.nix defines. This change reads it from there. We also have a NixOS module nixos/modules/monitoring/vpn/server.nix which reads this "input" and uses it to configure networking.wireguard.interfaces.monitoringvpn. This is fed onwards to to generate actual system configuration.

It seems better to use the former than the latter because we have more direct control over it and if we consider the whole configuration system a function then it is more like an argument we are supplying rather than an obscure implementation detail.