Implement status REST Call about ntopng

[NicoMaio]

For this new REST, I was thinking about "/lua/rest/v2/get/system/status.lua" as the endpoint
and a JSON Object as a response like this:

{ "license_info":{ "license":"", "status":"", "maintenance":"", "system_id":"" }, "resources_used":{ "system":{ "CPU_load":"", "CPU_states":{ "active":"", "iowait":"", "idle":"" }, "RAM":{ "used":"", "available":"", "total":"" } }, "ntopng":{ "RAM_used":"" } } }

[MatteoBiscosi]

Implemented as requested