It's not perfect but does the trick (for me)

* discover devices in subnets that responds to ping
* descover specified ports on found devices
* adds host group for each scanned subnet
* adds service goups
* restarts nagios

USAGE:
- place files in the nagios/etc folder
- edit :
  variables in nagios_discover-config.sh
  subnets in the function function discover() in nagios_discover_hosts.sh
  ports in the function function discover() in nagios_discover_services.sh
- run :
  nagios_discover.sh