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,

rdci/include/RdciSubSystem.h

@@ -39,7 +39,11 @@ class RdciSubSystem {
 
      virtual void process() = 0;
      virtual ~RdciSubSystem();
+
+     bool is_json_output() const;
+
  protected:
+     void set_json_output(bool is_json);
      std::vector<std::string> split_string(const std::string& s,
             char delimiter) const;
      void show_common_usage() const;
@@ -52,6 +56,9 @@ class RdciSubSystem {
      std::string root_ca_;
      std::string client_cert_;
      std::string client_key_;
+
+ private:
+     bool is_json_output_;
 };
 
 typedef std::shared_ptr<RdciSubSystem> RdciSubSystemPtr;