correct algorithm for deleting all images

when deleting all images, we need to iterate all the images deleting on those who dont
have children first. And then reiterate until they are all gone.

This resolves #1926

Signed-off-by: baude <bbaude@redhat.com>

cmd/podman/rmi.go

@@ -91,8 +91,23 @@ func rmiCmd(c *cli.Context) error {
 		if err != nil {
 			return errors.Wrapf(err, "unable to query local images")
 		}
-		for _, i := range imagesToDelete {
-			removeImage(i)
+		lastNumberofImages := 0
+		for len(imagesToDelete) > 0 {
+			if lastNumberofImages == len(imagesToDelete) {
+				return errors.New("unable to delete all images; re-run the rmi command again.")
+			}
+			for _, i := range imagesToDelete {
+				isParent, err := i.IsParent()
+				if err != nil {
+					return err
+				}
+				if isParent {
+					continue
+				}
+				removeImage(i)
+			}
+			lastNumberofImages = len(imagesToDelete)
+			imagesToDelete, err = runtime.ImageRuntime().GetImages()
 		}
 	} else {
 		// Create image.image objects for deletion from user input.