cmd/dist, images: allow image delete

This adds very simple deletion of images by name. We still need to
consider the approach to handling image name, so this may change. For
the time being, it allows one to delete an image entry in the metadata
database.

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

cmd/dist/main.go

@@ -62,6 +62,7 @@ distribution tool
 	}
 	app.Commands = []cli.Command{
 		imagesCommand,
+		rmiCommand,
 		pullCommand,
 		fetchCommand,
 		fetchObjectCommand,