controller: ensure deployment rollback is re-entrant

Make rollbacks re-entrant in the Deployment controller, otherwise
fast enqueues of a Deployment may end up in undesired behavior
- redundant rollbacks.

pkg/controller/deployment/deployment_controller.go

@@ -570,11 +570,11 @@ func (dc *DeploymentController) syncDeployment(key string) error {
 		return dc.sync(d)
 	}
 
+	// rollback is not re-entrant in case the underlying replica sets are updated with a new
+	// revision so we should ensure that we won't proceed to update replica sets until we
+	// make sure that the deployment has cleaned up its rollback spec in subsequent enqueues.
 	if d.Spec.RollbackTo != nil {
-		revision := d.Spec.RollbackTo.Revision
-		if d, err = dc.rollback(d, &revision); err != nil {
-			return err
-		}
+		return dc.rollback(d)
 	}
 
 	scalingEvent, err := dc.isScalingEvent(d)