builder/googlecompute: Use ssh_private_key_file if provided

This seemed to be missing from the googlecompute provider. Now if the
ssh_private_key_file is provided, that will be used in place of a temporary
key. I didn't update the googlecompute specific docs under `./website/`, since
this parameter is already documented under the communicators templates page.

builder/googlecompute/builder.go

@@ -54,6 +54,7 @@ func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packe
 		&StepCreateSSHKey{
 			Debug:          b.config.PackerDebug,
 			DebugKeyPath:   fmt.Sprintf("gce_%s.pem", b.config.PackerBuildName),
+			PrivateKeyFile: b.config.Comm.SSHPrivateKey,
 		},
 		&StepCreateInstance{
 			Debug: b.config.PackerDebug,

builder/googlecompute/step_create_instance.go

@@ -24,13 +24,17 @@ func (c *Config) createInstanceMetadata(sourceImage *Image, sshPublicKey string)
 		instanceMetadata[k] = v
 	}
 
-	// Merge any existing ssh keys with our public key.
+	// Merge any existing ssh keys with our public key, unless there is no
+	// supplied public key. This is possible if a private_key_file was
+	// specified.
+	if sshPublicKey != "" {
 		sshMetaKey := "sshKeys"
 		sshKeys := fmt.Sprintf("%s:%s", c.Comm.SSHUsername, sshPublicKey)
 		if confSshKeys, exists := instanceMetadata[sshMetaKey]; exists {
 			sshKeys = fmt.Sprintf("%s\n%s", sshKeys, confSshKeys)
 		}
 		instanceMetadata[sshMetaKey] = sshKeys
+	}
 
 	// Wrap any startup script with our own startup script.
 	if c.StartupScriptFile != "" {
@@ -65,9 +69,13 @@ func getImage(c *Config, d Driver) (*Image, error) {
 func (s *StepCreateInstance) Run(state multistep.StateBag) multistep.StepAction {
 	c := state.Get("config").(*Config)
 	d := state.Get("driver").(Driver)
-	sshPublicKey := state.Get("ssh_public_key").(string)
 	ui := state.Get("ui").(packer.Ui)
 
+	sshPublicKey := ""
+	if sshPublicKeyRaw, ok := state.GetOk("ssh_public_key"); ok {
+		sshPublicKey = sshPublicKeyRaw.(string)
+	}
+
 	sourceImage, err := getImage(c, d)
 	if err != nil {
 		err := fmt.Errorf("Error getting source image for instance creation: %s", err)

builder/googlecompute/step_create_ssh_key.go

@@ -6,6 +6,7 @@ import (
 	"crypto/x509"
 	"encoding/pem"
 	"fmt"
+	"io/ioutil"
 	"os"
 
 	"github.com/mitchellh/multistep"
@@ -17,6 +18,7 @@ import (
 type StepCreateSSHKey struct {
 	Debug          bool
 	DebugKeyPath   string
+	PrivateKeyFile string
 }
 
 // Run executes the Packer build step that generates SSH key pairs.
@@ -25,6 +27,20 @@ type StepCreateSSHKey struct {
 func (s *StepCreateSSHKey) Run(state multistep.StateBag) multistep.StepAction {
 	ui := state.Get("ui").(packer.Ui)
 
+	if s.PrivateKeyFile != "" {
+		ui.Say("Using existing SSH private key")
+		privateKeyBytes, err := ioutil.ReadFile(s.PrivateKeyFile)
+		if err != nil {
+			state.Put("error", fmt.Errorf(
+				"Error loading configured private key file: %s", err))
+			return multistep.ActionHalt
+		}
+
+		state.Put("ssh_private_key", string(privateKeyBytes))
+
+		return multistep.ActionContinue
+	}
+
 	ui.Say("Creating temporary SSH key for instance...")
 	priv, err := rsa.GenerateKey(rand.Reader, 2048)
 	if err != nil {