Allow to use a challenge element if there is only one challenge in a track

[danielmitterdorfer]

Some tracks only define one challenge:

    {
      "description": "....",
      "indices": [  ],
      "challenges": [
        {
          "name": "index-and-query",
          "description": "...",
          "schedule": [ ]
        }
      ]
    }

To simplify the syntax a bit, we should allow to use the following syntax in this case:

    {
      "description": "....",
      "indices": [  ],
      "challenge": {
          "name": "index-and-query",
          "description": "...",
          "schedule": [ ]
        }
    }

Note how we replaced the challenges array with a single challenge property.