title: Flink task slots are not evenly distributed when setting operator parallelism larger than default parallelism
body: <p>I'm running a Flink job on a cluster containing 3 task managers (on top of 3 Kubernetes pods).
  Job's default parallelism is 9 and one of the operators is set to parallelism 18.
  Job's number of task slot is set to 18 (the largest parallelism value).</p>
  
  <p>I observe the following behavior:</p>
  
  <p>The operator set to parallelism 18 is equally distributed between all task slots.</p>
  
  <p>All other operators (set to default - 9) are not distributed equally. For example:</p>
  
  <ul>
  <li>TM1: running 2 sub-tasks</li>
  <li>TM2: running 5 sub-tasks</li>
  <li>TM3: running 2 sub-tasks</li>
  </ul>
  
  <p>Can someone please explain the following - </p>
  
  <ul>
  <li>What causes this uneven distribution?</li>
  <li>Can I control operator assignment to be ballanced? how can I do it?</li>
  </ul>
  
  <p>(Running with Flink v1.6.3)</p>