Add option showing summarized result of benchmark in "System-radable"

Add new options ( --json ) for 'rally task detailed' comamnd.

Because The outside system of rally will sometimes want to use summarized result of benchmark.
But current Rally can only show summarized result of benchmark in "Human-readable" text by using following command 'rally task detailed'.
So If other system want to use summarized result of benachmark, we have to parse result of following command 'rally task detailed'.
Rally should have method of showing summarized result of benchmark in "System-readable" text

new option works like fiollowing

$rally task detailed --json
{
  "scenarios": [
    {
      "args_position": 0,
      "args_values": {
        "args": {
          "flavor": {
            "name": "m1.tiny"
          },
          "force_delete": false,
          "image": {
            "name": "^cirros.*"
          }
        },
        "context": {
          "users": {
            "project_domain": "default",
            "resource_management_workers": 30,
            "tenants": 3,
            "user_domain": "default",
            "users_per_tenant": 2
          }
        },
        "runner": {
          "concurrency": 2,
          "times": 10,
          "type": "constant"
        }
      },
      "full_duration": 48.877161502838135,
      "load_duration": 36.500731468200684,
      "scenario": "NovaServers.boot_and_delete_server",
      "ssrs_results": [],
      "summarized_results": [
        {
          "90%ile": 4.76,
          "95%ile": 4.78,
          "action": "nova.boot_server",
          "avg": 4.704,
          "count": 10,
          "max": 4.8,
          "median": 4.697,
          "min": 4.61,
          "success": "100.0%"
        },
        {
          "90%ile": 2.622,
          "95%ile": 2.645,
          "action": "nova.delete_server",
          "avg": 2.543,
          "count": 10,
          "max": 2.668,
          "median": 2.53,
          "min": 2.479,
          "success": "100.0%"
        },
        {
          "90%ile": 7.32,
          "95%ile": 7.343,
          "action": "total",
          "avg": 7.248,
          "count": 10,
          "max": 7.367,
          "median": 7.276,
          "min": 7.121,
          "success": "100.0%"
        }
      ]
    }
  ],
  "status": "finished",
  "task_id": "9954c8ed-0203-4656-adb7-3030425e3575"
}