GCP image handling enhancements

- the `public` option was not previously implemented
- add `replace` only for GCP images which will error otherwise. Only
  recommended for use in development, in production use the `--name` option
  to provide a different name eaxch time. Note only applies to GCP images,
  will document these options properly soon.
- add a `family` option; this allows you to upload many images and the
  user can select the latest using the `family` option instead of a specific
  image.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

config.go

@@ -29,7 +29,9 @@ type Moby struct {
 		Format  string
 		Project string
 		Bucket  string
+		Family  string
 		Public  bool
+		Replace bool
 	}
 }