Pass through cgroups when trying to run systemd

From https://discuss.circleci.com/t/docker-with-centos7-systemd-container/885/14

.circleci/integration-test.py

@@ -25,6 +25,7 @@ def run_systemd_image(image_name, container_name):
     subprocess.check_call([
         'docker', 'run',
         '--privileged',
+        '-v /sys/fs/cgroup:/sys/fs/cgroup',
         '--detach',
         '--name', container_name,
         image_name