Use /docker as cgroup parent instead of docker

It means that containers will be created under root cgroup and not under
daemon cgroup.

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

daemon/execdriver/native/template/default_template_linux.go

@@ -37,7 +37,7 @@ func New() *configs.Config {
 			{Type: "NEWUSER"},
 		}),
 		Cgroups: &configs.Cgroup{
-			Parent:           "docker",
+			Parent:           "/docker",
 			AllowAllDevices:  false,
 			MemorySwappiness: -1,
 		},