Snmp on linux

[algorys]

Add some tutorial for snmp in Linux:

• Debian
• Freebsd

[mohierf]

Some reusable material:

• https://github.com/mohierf/Monitoring-configuration/tree/master/doc (snmp, nrpe, wmi, ...)
• on https://github.com/Alignak-monitoring-contrib, all the alignak-checks contain a doc