feat(serialization): Add #cloud-config header

cloudinit/cloud_config.go

@@ -1,6 +1,7 @@
 package cloudinit
 
 import (
+	"fmt"
 	"log"
 
 	"launchpad.net/goyaml"
@@ -26,11 +27,14 @@ func NewCloudConfig(contents []byte) (*CloudConfig, error) {
 
 func (cc CloudConfig) String() string {
 	bytes, err := goyaml.Marshal(cc)
-	if err == nil {
-		return string(bytes)
-	} else {
+	if err != nil {
 		return ""
 	}
+
+	stringified := string(bytes)
+	stringified = fmt.Sprintf("#cloud-config\n%s", stringified)
+
+	return stringified
 }
 
 func ApplyCloudConfig(cfg CloudConfig, sshKeyName string) error {

cloudinit/cloud_config_test.go

@@ -1,6 +1,7 @@
 package cloudinit
 
 import (
+	"strings"
 	"testing"
 )
 
@@ -146,3 +147,12 @@ ssh_authorized_keys:
 		t.Error("Parsed incorrect number of SSH keys")
 	}
 }
+
+func TestCloudConfigSerializationHeader(t *testing.T) {
+	cfg, _ := NewCloudConfig([]byte{})
+	contents := cfg.String()
+	header := strings.SplitN(contents, "\n", 2)[0]
+	if header != "#cloud-config" {
+		t.Fatalf("Serialized config did not have expected header")
+	}
+}