Add support for cgroup namespace

Cgroup namespace can be configured in `config.json` as other
namespaces. Here is an example:

```
"namespaces": [
	{
		"type": "pid"
	},
	{
		"type": "network"
	},
	{
		"type": "ipc"
	},
	{
		"type": "uts"
	},
	{
		"type": "mount"
	},
	{
		"type": "cgroup"
	}
],

```

Note that if you want to run a container which has shared cgroup ns with
another container, then it's strongly recommended that you set
proper `CgroupsPath` of both containers(the second container's cgroup
path must be the subdirectory of the first one). Or there might be
some unexpected results.

Signed-off-by: Yuanhong Peng <pengyuanhong@huawei.com>
Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

libcontainer/README.md

@@ -148,6 +148,7 @@ config := &configs.Config{
 		{Type: configs.NEWPID},
 		{Type: configs.NEWUSER},
 		{Type: configs.NEWNET},
+		{Type: configs.NEWCGROUP},
 	}),
 	Cgroups: &configs.Cgroup{
 		Name:   "test-container",