Fixing and improving release labelling in rollback

1. Fixed propagating labels to rollback release
2. Added '-l' option to manage labels during rollback

Signed-off-by: Marcin Chojnacki <marcin.chojnacki@nokia.com>

cmd/helm/rollback.go

@@ -85,7 +85,7 @@ func newRollbackCmd(cfg *action.Configuration, out io.Writer) *cobra.Command {
 	f.BoolVar(&client.WaitForJobs, "wait-for-jobs", false, "if set and --wait enabled, will wait until all Jobs have been completed before marking the release as successful. It will wait for as long as --timeout")
 	f.BoolVar(&client.CleanupOnFail, "cleanup-on-fail", false, "allow deletion of new resources created in this rollback when rollback fails")
 	f.IntVar(&client.MaxHistory, "history-max", settings.MaxHistory, "limit the maximum number of revisions saved per release. Use 0 for no limit")
-	f.StringToStringVarP(&client.Labels, "labels", "l", nil, "Labels that would be added to release metadata. Should be divided by comma. By default labels are propagated from the rollback target release.")
+	f.StringToStringVarP(&client.Labels, "labels", "l", nil, "Labels that would be added to release metadata. Should be divided by comma. Labels from the rollback target release and will be merged with rollback labels. You can unset label using null.")
 
 	return cmd
 }