Use clusterloader metrics for test duration

[akrzos]

Where ever possible we should use cluster loader metrics to determine test duration. Previously cluster loader test duration metrics were in a format that made quick jq comparisons difficult, however now the duration metrics is simply in nanoseconds:

{
  "marker": "cluster_loader_marker",
  "name": "cluster-loader-test",
  "type": "metrics.TestDuration",
  "startTime": "2019-08-09T13:40:47.96451457Z",
  "testDuration": 56681396581,
  "Steps": [
    {
      "type": "metrics.PodStepDuration",
      "startTime": "2019-08-09T13:40:48.308574872Z",
      "totalTime": 38262069047,
      "waitPodsDurations": [
        31011157273
      ],
      "rateDelay": 0,
      "rateDelayCount": 0,
      "stepPause": 0,
      "stepPauseCount": 1,
      "syncTime": 200
    }
  ]
}

This would replace running date commands to determine duration (Example in nodevertical) with simply comparing against the value in the clusterloader.json