cmd/dist: port commands over to use GRPC content store

Following from the rest of the work in this branch, we now are porting
the dist command to work directly against the containerd content API.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

cmd/dist/main.go

@@ -42,6 +42,11 @@ distribution tool
 			Usage: "path to content store root",
 			Value: "/tmp/content", // TODO(stevvooe): for now, just use the PWD/.content
 		},
+		cli.StringFlag{
+			Name:  "socket, s",
+			Usage: "socket path for containerd's GRPC server",
+			Value: "/run/containerd/containerd.sock",
+		},
 	}
 	app.Commands = []cli.Command{
 		fetchCommand,