Move NewClient and AppContext to commands pkg

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

cmd/ctr/start.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"github.com/containerd/console"
+	"github.com/containerd/containerd/cmd/ctr/commands"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
 	"github.com/urfave/cli"
@@ -25,7 +26,7 @@ var taskStartCommand = cli.Command{
 		if id == "" {
 			return errors.New("container id must be provided")
 		}
-		client, ctx, cancel, err := newClient(context)
+		client, ctx, cancel, err := commands.NewClient(context)
 		if err != nil {
 			return err
 		}