Provide default args: percpu=true and totalcpu=true for cpu plugin

Also if outputs.file is empty, write to stdout

closes #720

plugins/inputs/system/cpu.go

@@ -113,6 +113,10 @@ func totalCpuTime(t cpu.CPUTimesStat) float64 {
 
 func init() {
 	inputs.Add("cpu", func() telegraf.Input {
-		return &CPUStats{ps: &systemPS{}}
+		return &CPUStats{
+			PerCPU:   true,
+			TotalCPU: true,
+			ps:       &systemPS{},
+		}
 	})
 }