cpu_percent missing from NodesStats

[gmarz]

Follow up from #2180

      "os" : {
        "timestamp" : 1469805611911,
        **"cpu_percent" : 18,**
        "load_average" : -1.0,
        "mem" : {
          "total_in_bytes" : 16843710464,
          "free_in_bytes" : 10167721984,
          "used_in_bytes" : 6675988480,
          "free_percent" : 60,
          "used_percent" : 40
        },
        "swap" : {
          "total_in_bytes" : 19930718208,
          "free_in_bytes" : 11793481728,
          "used_in_bytes" : 8137236480
        }
      }

This applies to 2.x only. /cc @TheFireCookie

[gmarz]

Closed via #2186