include std_deviation in json formatter

[jtluka]

Currently the formatted json data provides only average of each metric:

    {                                                                           
        "result": "PASS",                                                       
        "type": "measurement",                                                  
        "measurement_type": "cpu",                                              
        "data": {                                                               
            "utilization": 86.636369661039                                                                                                                                                     
        }                                                                       
    },                                                                          

We should format this so that the data includes both average and std_deviation:

    {                                                                           
        "result": "PASS",                                                       
        "type": "measurement",                                                  
        "measurement_type": "cpu",                                              
        "data": {                                                               
            "utilization": {
                "average": 86.636369661039,                                                                                                                                                     
                "std_deviation": 2.1,
        }                                                                       
    },