Parallelize unit tests on the pytest level

It won't be a huge gain, maybe only 5 minutes in total,
because we are targeting 3 to 4 minutes jobs,
and travis runners have only 2 cores.

PR ansible#1706. Ref ansible#1669.

Signed-off-by: Fabrice Flore-Thebault <themr0c@users.noreply.github.com>

tox.ini

@@ -41,7 +41,7 @@ extras =
     openstack
     vagrant
 commands =
-    unit: pytest test/unit/ --cov={toxinidir}/molecule/ --no-cov-on-fail {posargs}
+    unit: pytest test/unit/ --cov={toxinidir}/molecule/ --no-cov-on-fail -n auto {posargs}
     functional: pytest test/functional/ {posargs}
     lint: flake8
     lint: yamllint -s test/ molecule/