Update gopsutil code

Latest gosutil includes two backward incompatible changes:

First, it removed unused Stolen field in
shirou/gopsutil@cae8efc#diff-d9747e2da342bdb995f6389533ad1a3d
.

Second, it updated the Windows cpu stats calculation to be inline with
other platforms, where it returns absolate stats rather than
percentages.  See shirou/gopsutil#611.

client/stats/cpu.go

@@ -5,6 +5,7 @@ import (
 	"time"
 
 	shelpers "github.com/hashicorp/nomad/helper/stats"
+	"github.com/shirou/gopsutil/cpu"
 )
 
 // CpuStats calculates cpu usage percentage
@@ -59,3 +60,31 @@ func (c *CpuStats) calculatePercent(t1, t2 float64, timeDelta int64) float64 {
 	overall_percent := (vDelta / float64(timeDelta)) * 100.0
 	return overall_percent
 }
+
+func (h *HostStatsCollector) collectCPUStats() (cpus []*CPUStats, totalTicks float64, err error) {
+
+	ticksConsumed := 0.0
+	cpuStats, err := cpu.Times(true)
+	if err != nil {
+		return nil, 0.0, err
+	}
+	cs := make([]*CPUStats, len(cpuStats))
+	for idx, cpuStat := range cpuStats {
+		percentCalculator, ok := h.statsCalculator[cpuStat.CPU]
+		if !ok {
+			percentCalculator = NewHostCpuStatsCalculator()
+			h.statsCalculator[cpuStat.CPU] = percentCalculator
+		}
+		idle, user, system, total := percentCalculator.Calculate(cpuStat)
+		cs[idx] = &CPUStats{
+			CPU:    cpuStat.CPU,
+			User:   user,
+			System: system,
+			Idle:   idle,
+			Total:  total,
+		}
+		ticksConsumed += (total / 100.0) * (shelpers.TotalTicksAvailable() / float64(len(cpuStats)))
+	}
+
+	return cs, ticksConsumed, nil
+}