text: Title: Don't show worker nodes when pytest is run quiet
  Body: I have a manycore machine (32 threads), and when I use pytest-xdist, the output of the worker node status is a bit bothersome due to showing 32 node status messages multiple times.
  
  This is the output of running pytest-xdist with a subset of the mypy test suite (a full screen CMD window on a 4k monitor):
  ![image](https://user-images.githubusercontent.com/9504279/48320945-924a4080-e5d3-11e8-9deb-b5588784a7f2.png)
  
  The setup messages take up about 90% of the window.
  
  It would be nice if this could be hidden when pytest is run in quiet mode, as it makes the time to print out the test setup info non-trivial (in addition it fills up the screen making scroll back more annoying).