Fix default cgroup path

Alternative of #895 , part of #892

The intension of current behavior if to create cgroup in
parent cgroup of current process, but we did this in a
wrong way, we used devices cgroup path of current process
as the default parent path for all subsystems, this is
wrong because we don't always have the same cgroup path
for all subsystems.

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>

libcontainer/specconv/spec_linux_test.go

@@ -3,7 +3,6 @@
 package specconv
 
 import (
-	"strings"
 	"testing"
 
 	"github.com/opencontainers/runtime-spec/specs-go"
@@ -33,7 +32,7 @@ func TestLinuxCgroupsPathNotSpecified(t *testing.T) {
 		t.Errorf("Couldn't create Cgroup config: %v", err)
 	}
 
-	if !strings.HasSuffix(cgroup.Path, "/ContainerID") {
-		t.Errorf("Wrong cgroupsPath, expected it to have suffix '%s' got '%s'", "/ContainerID", cgroup.Path)
+	if cgroup.Path != "" {
+		t.Errorf("Wrong cgroupsPath, expected it to be empty string, got '%s'", cgroup.Path)
 	}
 }