Fixes #1092, Adds a format option to the {{isotime}} variable.

Now using the golang magic date: Mon Jan 2 15:04:05 -0700 MST 2006
One can format the time like:

    {{isotime "2006-01-02"}} == "YYYY-MM-DD"
    {{isotime "060102-15"}}  == "YYMMDD-HH" (24-hour clock)
    {{isotime "060102-3"}}   == "YYMMDD-H"  (12-hour clock)

Using {{isotime}} as a standalone variable doesn't change. It still returns RFC3339 formatted time.

packer/config_template.go

@@ -110,8 +110,16 @@ func templateEnv(n string) string {
 	return os.Getenv(n)
 }
 
-func templateISOTime() string {
-	return time.Now().UTC().Format(time.RFC3339)
+func templateISOTime(timeFormat ...string) (string, error) {
+	if len(timeFormat) == 0 {
+		return time.Now().UTC().Format(time.RFC3339), nil
+	}
+	
+	if len(timeFormat) > 1 {
+		return "", fmt.Errorf("too many values, 1 needed: %v", timeFormat)
+	}
+	
+	return time.Now().UTC().Format(timeFormat[0]), nil
 }
 
 func templatePwd() (string, error) {

packer/config_template_test.go

@@ -1,6 +1,7 @@
 package packer
 
 import (
+	"fmt"
 	"math"
 	"os"
 	"strconv"
@@ -42,6 +43,33 @@ func TestConfigTemplateProcess_isotime(t *testing.T) {
 	}
 }
 
+// Note must format with the magic Date: Mon Jan 2 15:04:05 -0700 MST 2006
+func TestConfigTemplateProcess_isotime_withFormat(t *testing.T) {
+	tpl, err := NewConfigTemplate()
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	// Checking for a too-many arguments error
+	// Because of the variadic function, compile time checking won't work
+	_, err = tpl.Process(`{{isotime "20060102" "huh"}}`, nil)
+	if err == nil {
+		t.Fatalf("err: cannot have more than 1 input")
+	}
+	
+	result, err := tpl.Process(`{{isotime "20060102"}}`, nil)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+	
+	ti := time.Now().UTC() 
+	val := fmt.Sprintf("%04d%02d%02d", ti.Year(), ti.Month(), ti.Day()) 
+
+	if result != val {
+		t.Fatalf("val: %s (formated: %s)", val, result)
+	}
+}
+
 func TestConfigTemplateProcess_pwd(t *testing.T) {
 	tpl, err := NewConfigTemplate()
 	if err != nil {