title: How to I configure tox so it will run pytest coverage on a single environment instead of all?
body: <p>I do have a complex tox.ini configuration with multiple environments for different versions of python.</p>
  
  <p>I would like to know how to tell tox to run coverage only on the default python interpretor.</p>
  
  <p>One of the problems is that the default python environment can be different from one platform to another.</p>
  
  <p>I do have a wrapper script which calls <code>tox -e py25,py26,docs</code> where the -e arguments are the detected versions of python.</p>
  
  <pre><code>[tox]
       ...
  [testenv:docs]
       ...
  [testenv]
       commands=py.test --cov-report xml --cov scripts
       ...
  [testenv:py26]
       ...
  [testenv:py25]
       ...
  </code></pre>
  
  <p>Desired behaviour: run pytest with coverage for a single environment (this is supposed to run integrated with jenkins).</p>