Add --cgroup_root flag to Kubelet to set the parent cgroup for all pods.

Implements #6991.

pkg/kubelet/container/runtime.go

@@ -201,6 +201,8 @@ type RunContainerOptions struct {
 	// into docker's container runtime.
 	NetMode string
 	IpcMode string
+	// The parent cgroup to pass to Docker
+	CgroupParent string
 }
 
 type Pods []*Pod