added the wwctl overlay parse command

The `wwctl overlay parse -n NODE FILE` will interpret the given overlay file and print it stdout, via wwlog.PRINTF.
The overlay code had to be refactored for this, but hopefully this makes the code a bit more readable. Also the tstruct has not its own initilization code.

internal/pkg/overlay/overlay.go

@@ -243,12 +243,15 @@ func BuildOverlayIndir(nodeInfo node.NodeInfo, overlayNames []string, outputDir
 				destFile := strings.TrimSuffix(location, ".ww")
 
 				buffer, backupFile, writeFile, err := RenderTemplateFile(location, tstruct)
+				if err != nil {
+					return errors.Wrap(err, fmt.Sprintf("Failed to render template %s", location))
+				}
 				if writeFile {
 					destFileName := destFile
 					var fileBuffer bytes.Buffer
 					// search for magic file name comment
 					fileScanner := bufio.NewScanner(bytes.NewReader(buffer.Bytes()))
-					fileScanner.Split(scanLines)
+					fileScanner.Split(ScanLines)
 					reg := regexp.MustCompile(`.*{{\s*/\*\s*file\s*["'](.*)["']\s*\*/\s*}}.*`)
 					foundFileComment := false
 					for fileScanner.Scan() {
@@ -346,7 +349,11 @@ Parses the template with the given filename, variables must be in data. Returns
 parsed template as bytes.Buffer, and the bool variables for backupFile and writeFile.
 If something goes wrong an error is returned.
 */
-func RenderTemplateFile(fileName string, data TemplateStruct) (buffer bytes.Buffer, backupFile bool, writeFile bool, err error) {
+func RenderTemplateFile(fileName string, data TemplateStruct) (
+	buffer bytes.Buffer,
+	backupFile bool,
+	writeFile bool,
+	err error) {
 	backupFile = true
 	writeFile = true
 	tmpl, err := template.New(path.Base(fileName)).Option("missingkey=default").Funcs(template.FuncMap{
@@ -385,7 +392,7 @@ func RenderTemplateFile(fileName string, data TemplateStruct) (buffer bytes.Buff
 }
 
 // Simple version of ScanLines, but include the line break
-func scanLines(data []byte, atEOF bool) (advance int, token []byte, err error) {
+func ScanLines(data []byte, atEOF bool) (advance int, token []byte, err error) {
 	if atEOF && len(data) == 0 {
 		return 0, nil, nil
 	}