[daemon] Refactor image_delete.go

This file was not well documented and had very high cyclomatic complexity.
This patch completely rearranges this file and the ImageDelete method to
be easier to follow and more maintainable in the future.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

api/server/image.go

@@ -233,10 +233,15 @@ func (s *Server) deleteImages(version version.Version, w http.ResponseWriter, r
 	}
 
 	name := vars["name"]
-	force := boolValue(r, "force")
-	noprune := boolValue(r, "noprune")
 
-	list, err := s.daemon.ImageDelete(name, force, noprune)
+	if name == "" {
+		return fmt.Errorf("image name cannot be blank")
+	}
+
+	force := boolValue(r, "force")
+	prune := !boolValue(r, "noprune")
+
+	list, err := s.daemon.ImageDelete(name, force, prune)
 	if err != nil {
 		return err
 	}