Add ability to send log output to a file

Using PACKER_LOG=1 causes all the log output to be sent to Stderr. This
change maintains that backward compatility. Anything other than "1" will
be treated as a filename which will have logging appended to that file.
This is useful, for example, to always have debugging available without
cluttering up stdout (and without having to redirect stderr all the time).

packer.go

@@ -14,12 +14,25 @@ import (
 )
 
 func main() {
-	if os.Getenv("PACKER_LOG") == "" {
+	switch packer_log := os.Getenv("PACKER_LOG"); packer_log {
+	case "":
 		// If we don't have logging explicitly enabled, then disable it
 		log.SetOutput(ioutil.Discard)
-	} else {
-		// Logging is enabled, make sure it goes to stderr
+	case "1":
+		// Legacy logging is enabled, make sure it goes to stderr
 		log.SetOutput(os.Stderr)
+	default:
+		{
+			// Use a file for logging
+			file, err := os.OpenFile(packer_log, os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0600)
+			if err == nil {
+				log.SetOutput(file)
+			} else {
+				// Problem opening the file, fail back to Stderr
+				log.SetOutput(os.Stderr)
+				log.Printf("Could not open %s for logging (%s). Using stderr instead.", packer_log, err.Error())
+			}
+		}
 	}
 
 	// If there is no explicit number of Go threads to use, then set it