Support standard deviation and json output for job stats

In the job stats, in addition to the max, min and average,
it will also display the standard deviation.

A new option --json is added to the rdci to output the results
in json format.

In the job stats, using the GMT time instead of timestamp
for start and end time.

Change-Id: If245c4fc4854a1dc867f97ff5aa9112af7962eca

rdci/include/RdciStatsSubSystem.h

@@ -39,6 +39,7 @@ class RdciStatsSubSystem: public RdciSubSystem {
  private:
      void show_help() const;
      void show_job_stats(const rdc_gpu_usage_info_t& gpu_info) const;
+     void show_job_stats_json(const rdc_gpu_usage_info_t& gpu_info) const;
 
      enum OPERATIONS {
         STATS_UNKNOWN = 0,