Accept extra positional arguments in tox

Example:

    tox -e py35-ansible22-unit -- --collect-only

tox.ini

@@ -12,8 +12,8 @@ deps =
     ansible22: ansible~=2.2
 
 commands =
-    unit: pytest
-    flake8: flake8
+    unit: pytest {posargs}
+    flake8: flake8 {posargs}
     pylint: python setup.py lint
     yamllint: python setup.py yamllint
     generate_validation: python setup.py generate_validation

utils/tox.ini

@@ -14,6 +14,6 @@ commands =
     # Needed to make detox work, since it ignores usedevelop
     # https://github.com/tox-dev/tox/issues/180
     unit: pip install -e .
-    unit: pytest
+    unit: pytest {posargs}
     flake8: python setup.py flake8
     pylint: python setup.py lint