Let swarmkit handle cluster defaults in swarm init if not specified

This fix tries to address the issue raised in 24958 where previously
`docker swarm init` will automatically fill in all the default value
(instead of letting swarmkit to handle the default).

This fix update the `swarm init` so that initial value are passed only
when a flag change has been detected.

This fix fixes 24958.

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

command/swarm/init.go

@@ -59,7 +59,7 @@ func runInit(dockerCli *command.DockerCli, flags *pflag.FlagSet, opts initOption
 		ListenAddr:      opts.listenAddr.String(),
 		AdvertiseAddr:   opts.advertiseAddr,
 		ForceNewCluster: opts.forceNewCluster,
-		Spec:            opts.swarmOptions.ToSpec(),
+		Spec:            opts.swarmOptions.ToSpec(flags),
 	}
 
 	nodeID, err := client.SwarmInit(ctx, req)