switch needed to enable --skip-unfound-ifaces to check_netlinks.py

When we enable 'netlinks' checks in NRPE, by default we will receive an alert for every interface not found by the system. In some cases, where we use a common nrpe application across multiple hosts, it would be good to not get an alert if, say, bond1 is missing because this is a KVM where there is no bond interfaces at all.

We could add an option to config.yaml that adds --skip-unfound-ifaces to the command options sent to check_netlinks.py