Add config file

This adds a config file for containerd configuration.  It is hard to
have structure data on cli flags and the config file should be used for
the majority of fields when configuring containerd.

There are still a few flags on the daemon that override config file
values but flags should take a back seat going forward and should be
kept at a minimum.

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

cmd/ctr/pprof.go

@@ -23,7 +23,7 @@ var pprofCommand = cli.Command{
 		cli.StringFlag{
 			Name:  "debug-socket, d",
 			Usage: "socket path for containerd's debug server",
-			Value: "/run/containerd/containerd-debug.sock",
+			Value: "/run/containerd/debug.sock",
 		},
 	},
 	Subcommands: []cli.Command{