Refactor task config format

(1) Rename the task config elements is one step in the process of making scenarios more pluggable.

Proposed schema.
{
   {
    "NovaServers.boot_and_delete_server": [
        {
            "args": {
                 "flavor_id": 1,
                 "image_id": "73257560-c59b-4275-a1ec-ab140e5b9979"
            },
            "runner": {
                "type": "continuous",
                "times": 10,
                “timeout”: 100,
                "active_users": 2
            }
            "context": {
                "users": {
                    "tenants": 3,
                    “users_per_tenant": 2
                }
            }
        }
    ]
}

(2) Support configuration in both yaml + json format.

(3) Use jsonschema to validate configuration.

More details in section 3.1 of Rally Benchmark Engine refactoring doc [1]

[1]: https://docs.google.com/a/yahoo-inc.com/document/d/1LYUAHkZQD8W7dtlj2I3PDA6x67TiD3AMnSWG6ljsups/edit#