[enhancement] Spilt netjson-monitoring in small plugin parts

[devkapilbansal]

This change was requested here: :point_down: https://github.com/openwisp/openwrt-openwisp-monitoring/pull/25#issuecomment-846430520

In general I would split the netjson into small plugin parts. The problem I see here is that not all have all related information on the system, so we should not check on every run if we have a wlan or a cellular. I do not have for example a cellular on my system. So why should the openwisp-monitoring always try to check the cellular backend.