Allow parallel tasks to exit on any completion

[b-deam]

With this commit we add the ability to specify a new any value
for the completed-by param for parallel task blocks. This
allows the parallel block to be completed by whichever task is
first to complete successfully.

Closes #1261

Tested with track:

{
  "description": "'any' testing ",
  "challenges": [
    {
      "name": "parallel-any",
      "description": "Track completed-by property",
      "schedule": [
        {
          "name": "outside-cluster-health",
          "operation": {
            "operation-type": "cluster-health"
          }
        },
        {
          "parallel": {
            "completed-by": "any",
            "tasks": [
              {
                "name": "check-cluster-health",
                "operation": {
                  "operation-type": "cluster-health"
                }
              },
              {
                "name": "run-node-stats",
                "operation": {
                  "operation-type": "node-stats"
                },
                "iterations": 300000000
              }
            ]
          }
        }
      ]
    }
  ]
}

[danielmitterdorfer]

Thanks for the PR! I left a few suggestions. IMHO also a test for the track loader is missing.

[b-deam]

I think I addressed the comments, can you take a look when you get a chance?

[danielmitterdorfer]

Thanks for iterating. I left one nit but other than that LGTM. No need for another review round.