Run tasks indefinitely

[danielmitterdorfer]

Consider the following schedule:

{
  "parallel": {
    "completed-by": "index",
    "warmup-time-period": 240,
    "tasks": [
      {
        "operation": "index",
        "clients": 8
      },
      {
        "operation": "term",
        "clients": 1,
        "target-throughput": 20
      }
    ]
  }
}

Expected behavior: We want to run a term query in parallel with indexing. However, we don't really care how often the term query runs. It should just run as long as we're indexing and then terminate.
Actual behavior: Rally runs the query just once and does not show any results because this sample is finished within the warmup time period of 240 seconds.

We should fix this by assuming that the term query should be run indefinitely. Note that with the new behavior we rely that it is only used in schedules like the above. For example, the following schedule will never terminate:

{
  "operation": "term",
  "clients": 1,
  "warmup-time-period": 240,
  "target-throughput": 20
}

If we want it to terminate, we need to specify a time-period.

For the time being we will not implement any validation for these cases.