Default parallelism to 1.

Signed-off-by: Daniel Nephin <dnephin@docker.com>
(cherry picked from commit 7c1b39923505adeb13512d6294855b31102963a6)
Signed-off-by: Victor Vieux <victorvieux@gmail.com>

cli/command/stack/deploy.go

@@ -571,8 +571,12 @@ func convertUpdateConfig(source *composetypes.UpdateConfig) *swarm.UpdateConfig
 	if source == nil {
 		return nil
 	}
+	parallel := uint64(1)
+	if source.Parallelism != nil {
+		parallel = *source.Parallelism
+	}
 	return &swarm.UpdateConfig{
-		Parallelism:     source.Parallelism,
+		Parallelism:     parallel,
 		Delay:           source.Delay,
 		FailureAction:   source.FailureAction,
 		Monitor:         source.Monitor,