Factoring improvements (VPN client, monitoring exporters)

This includes almost everything from and supersedes !237 (closed). Differences:

• Rebased on current develop.
• A little bit of reordering to clean the diff to develop.
• Changed factoring: I removed the one line about monitoring from the new file that is almost exclusively about the vpn network setup. It is now called monitoringvpn-client.nix instead of monitored-node.nix.