Add force option to service update

Currently, there's no way to restart the tasks of a service without
making an actual change to the service. This leads to us giving awkward
workarounds as in
https://github.com/docker/docker.github.io/pull/178/files, where we tell
people to scale a service up and down to restore balance, or make
unnecessary changes to trigger a restart.

This change adds a --force option to "docker service update", which
forces the service to be updated even if no changes require that.

Since rolling update parameters are respected, the user can use
"docker service --force" to do a rolling restart. For example, the
following is supported:

   docker service update --force --update-parallelism 2 \
   --update-delay 5s myservice

Since the default value of --update-parallelism is 1, the default
behavior is to restart the service one task at a time.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

command/service/update.go

@@ -37,6 +37,7 @@ func newUpdateCommand(dockerCli *command.DockerCli) *cobra.Command {
 	flags.String("image", "", "Service image tag")
 	flags.String("args", "", "Service command args")
 	flags.Bool("rollback", false, "Rollback to previous specification")
+	flags.Bool("force", false, "Force update even if no changes require it")
 	addServiceFlags(cmd, opts)
 
 	flags.Var(newListOptsVar(), flagEnvRemove, "Remove an environment variable")
@@ -257,6 +258,15 @@ func updateService(flags *pflag.FlagSet, spec *swarm.ServiceSpec) error {
 		return err
 	}
 
+	force, err := flags.GetBool("force")
+	if err != nil {
+		return err
+	}
+
+	if force {
+		spec.TaskTemplate.ForceUpdate++
+	}
+
 	return nil
 }