munin: refactor package and add nixos service

Minimal configuration to test:

```
services = {
    munin-node.enable = true;
    munin-cron = {
      enable = true;
      hosts = ''
        [${config.networking.hostName}]
        address localhost
      '';
      extraGlobalConfig = ''
        contact.email.command mail -s "Munin notification for ''${var:host}" domen@dev.si
      '';
    };
};
```

Wait up to 5min and open `/var/www/munin/`.