title: pytest.ini value not being set correctly
body: <p>Trying to set an option in a pytest.ini file dynamically. The option testpaths determines what directories pytest will gather tests from. I want to give users the ability to select test directory <code>a</code> or <code>b</code>. </p>
  
  <p><strong>conftest.py</strong></p>
  
  <p>The first hook creates a parser option. 
  The second hook pulls the parser option reads the value and adds the testpath to the testpaths option under the config object.</p>
  
  <pre><code>@pytest.hookimpl() 
  def pytest_addoption(parser):
      """Creates a parser option"""
  
      # Allows the user to select the test suite they want to run
      parser.addoption("--suite", action="store", default="None"
                       , choices=['a', 'b']
                       , help="Choose which test suite to run.")
  
  @pytest.hookimpl()
  def pytest_configure(config):
      print("Determining test directory")
      suite = config.getoption("--suite")
  
      if suite == "a":
          config.addinivalue_line("testpaths", "tests/a")
  
      elif suite == "b":
          config.addinivalue_line("testpaths", "tests/b")
  </code></pre>
  
  <p>So if i run <code>pytest --suite a</code> it should load all tests under the a test suite. It does not. It loads all tests like the option doesnt exist. </p>