Label containers with stack name when deploying stack/bundle

This fix tries to address the issue raised in 24881 where
the stack name is not visable when inspecting containers
deployed through stacks.

This fix adds `labelNamespace` (`com.docker.stack.namespace`)
to containers at the client side so that the stack name label
will show up after containers are deployed.

This fix is tested manually:
 - Build the binary with `make DOCKER_EXPERIMENTAL=1`
 - Create a bundle file and deploy with the `docker stack deploy`
 - Verify that `com.docker.stack.namespace` has been
set properly by checking contaners with `docker inspect <ContainerID>`

This fix fixes 24881.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

api/client/stack/deploy.go

@@ -168,6 +168,10 @@ func deployServices(
 					Command: service.Command,
 					Args:    service.Args,
 					Env:     service.Env,
+					// Service Labels will not be copied to Containers
+					// automatically during the deployment so we apply
+					// it here.
+					Labels: getStackLabels(namespace, nil),
 				},
 			},
 			EndpointSpec: &swarm.EndpointSpec{