Use namespace in default cgroup path

By default, the generated spec will place containers in cgroups by their
ids, we need to use the namespace as the cgroup root to avoid
containers with the same name being placed in the same cgroup.

```
11:perf_event:/to/redis
10:freezer:/to/redis
9:memory:/to/redis
8:devices:/to/redis
7:net_cls,net_prio:/to/redis
6:pids:/to/redis
5:hugetlb:/to/redis
4:cpuset:/to/redis
3:blkio:/to/redis
2:cpu,cpuacct:/to/redis
1:name=systemd:/to/redis

11:perf_event:/te/redis
10:freezer:/te/redis
9:memory:/te/redis
8:devices:/te/redis
7:net_cls,net_prio:/te/redis
6:pids:/te/redis
5:hugetlb:/te/redis
4:cpuset:/te/redis
3:blkio:/te/redis
2:cpu,cpuacct:/te/redis
1:name=systemd:/te/redis
```

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

docs/getting-started.md

@@ -196,18 +196,11 @@ func redisExample() error {
         }
         log.Printf("Successfully pulled %s image\n", image.Name())
 
-        spec, err := containerd.GenerateSpec(ctx, client, nil, containerd.WithImageConfig(image))
-        if err != nil {
-                return err
-        }
-        log.Printf("Successfully generate an OCI spec version %s based on %s image", spec.Version, image.Name())
-
         container, err := client.NewContainer(
                 ctx,
                 "redis-server",
-                containerd.WithSpec(spec),
-                containerd.WithImage(image),
                 containerd.WithNewSnapshot("redis-server-snapshot", image),
+                containerd.WithNewSpec(containerd.WithImageConfig(image)),
         )
         if err != nil {
                 return err