Place containerd inside cgroup

This adds a config option to place the `containerd` daemon process into
a cgroup so that proper resource usage and accounting can be applied.

It defaults to not being place inside a cgroup and will create a new
cgroup if the `path` does not exist in the config or join an existing
`path` if it already exists.

```toml
[cgroup]
    path = "/containerd"
```

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

server/config.go

@@ -25,6 +25,8 @@ type Config struct {
 	Subreaper bool `toml:"subreaper"`
 	// OOMScore adjust the containerd's oom score
 	OOMScore int `toml:"oom_score"`
+	// Cgroup specifies cgroup information for the containerd daemon process
+	Cgroup CgroupConfig `toml:"cgroup"`
 
 	md toml.MetaData
 }
@@ -46,6 +48,10 @@ type MetricsConfig struct {
 	Address string `toml:"address"`
 }
 
+type CgroupConfig struct {
+	Path string `toml:"path"`
+}
+
 // Decode unmarshals a plugin specific configuration by plugin id
 func (c *Config) Decode(id string, v interface{}) (interface{}, error) {
 	data, ok := c.Plugins[id]