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/container_linux.go

@@ -1745,7 +1745,6 @@ func (c *linuxContainer) currentState() (*State, error) {
 // can setns in order.
 func (c *linuxContainer) orderNamespacePaths(namespaces map[configs.NamespaceType]string) ([]string, error) {
 	paths := []string{}
-
 	for _, ns := range configs.NamespaceTypes() {
 
 		// Remove namespaces that we don't need to join.