[docker] Kubernetes support (new cgroups path)

On Kubernetes, container files are directly at the top level of cgroups,
instead of being in a directory (`docker` or `lxc`).

checks.d/docker.py

@@ -422,6 +422,9 @@ def _find_cgroup_filename_pattern(self):
                     return os.path.join('%(mountpoint)s/docker/%(id)s/%(file)s')
                 elif os.path.exists(stat_file_path_coreos):
                     return os.path.join('%(mountpoint)s/system.slice/docker-%(id)s.scope/%(file)s')
+                # Kubernetes, we can't test the existence of the path without the id
+                else:
+                    return os.path.join('%(mountpoint)s/%(id)s/%(file)s')
 
         raise Exception("Cannot find Docker cgroup directory. Be sure your system is supported.")