Change log to Ui.Warn and restore the removed functionality for now

command/push.go

@@ -3,7 +3,6 @@ package command
 import (
 	"fmt"
 	"io"
-	"log"
 	"os"
 	"os/signal"
 	"path/filepath"
@@ -55,7 +54,7 @@ func (c *PushCommand) Run(args []string) int {
 	}
 
 	if message != "" {
-		log.Printf("[WARN] -m/-message is deprecated and will be removed in a future Packer release")
+		c.Ui.Warn("[DEPRECATED] -m/-message is deprecated and will be removed in a future Packer release")
 	}
 
 	args = f.Args()
@@ -191,6 +190,9 @@ func (c *PushCommand) Run(args []string) int {
 
 	// Add the upload metadata
 	metadata := make(map[string]interface{})
+	if message != "" {
+		metadata["message"] = message
+	}
 	metadata["template"] = tpl.RawContents
 	metadata["template_name"] = filepath.Base(args[0])
 	uploadOpts.Metadata = metadata