Allow timeouts to be configured in config

This adds a singleton `timeout` package that will allow services and user
to configure timeouts in the daemon.  When a service wants to use a
timeout, it should declare a const and register it's default value
inside an `init()` function for that package.  When the default config
is generated, we can use the `timeout` package to provide the available
timeout keys so that a user knows that they can configure.

These show up in the config as follows:

```toml
[timeouts]
  "io.containerd.timeout.shim.cleanup" = 5
  "io.containerd.timeout.shim.load" = 5
  "io.containerd.timeout.shim.shutdown" = 3
  "io.containerd.timeout.task.state" = 2

```

Timeouts in the config are specified in seconds.

Timeouts are very hard to get right and giving this power to the user to
configure things is a huge improvement.  Machines can be faster and
slower and depending on the CPU or load of the machine, a timeout may
need to be adjusted.

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

cmd/containerd/command/config.go

@@ -22,6 +22,7 @@ import (
 	"os"
 
 	"github.com/BurntSushi/toml"
+	"github.com/containerd/containerd/pkg/timeout"
 	"github.com/containerd/containerd/services/server"
 	srvconfig "github.com/containerd/containerd/services/server/config"
 	"github.com/urfave/cli"
@@ -68,6 +69,12 @@ var configCommand = cli.Command{
 						config.Plugins[p.URI()] = p.Config
 					}
 				}
+				timeouts := timeout.All()
+				config.Timeouts = make(map[string]string)
+				for k, v := range timeouts {
+					config.Timeouts[k] = v.String()
+				}
+
 				_, err = config.WriteTo(os.Stdout)
 				return err
 			},