builder/googlecompute: Adds ability to specify service account

This commit allows user to specify the service account they want
to associate with the virtual machine provisionned by setting
the service_account_email field in the config.

It allows to manage permissions of the instantiated VM properly,
using a service account that can be tied up to IAM roles and
permissions.

builder/googlecompute/config.go

@@ -58,6 +58,8 @@ type Config struct {
 	UseInternalIP        bool              `mapstructure:"use_internal_ip"`
 	Zone                 string            `mapstructure:"zone"`
 
+	ServiceAccountEmail string `mapstructure:"service_account_email"`
+
 	Account            AccountFile
 	stateTimeout       time.Duration
 	imageAlreadyExists bool

builder/googlecompute/driver.go

@@ -75,6 +75,7 @@ type InstanceConfig struct {
 	OnHostMaintenance string
 	Preemptible       bool
 	Region            string
+	ServiceAccountEmail string
 	Scopes            []string
 	Subnetwork        string
 	Tags              []string

builder/googlecompute/driver_gce.go

@@ -343,6 +343,14 @@ func (d *driverGCE) RunInstance(c *InstanceConfig) (<-chan error, error) {
 		guestAccelerators = append(guestAccelerators, ac)
 	}
 
+	serviceAccount := &compute.ServiceAccount{
+		Email:  "default",
+		Scopes: c.Scopes,
+	}
+	if c.ServiceAccountEmail != "" {
+		serviceAccount.Email = c.ServiceAccountEmail
+	}
+
 	// Create the instance information
 	instance := compute.Instance{
 		Description: c.Description,
@@ -379,10 +387,7 @@ func (d *driverGCE) RunInstance(c *InstanceConfig) (<-chan error, error) {
 			Preemptible:       c.Preemptible,
 		},
 		ServiceAccounts: []*compute.ServiceAccount{
-			{
+			serviceAccount,
-				Email:  "default",
-				Scopes: c.Scopes,
-			},
 		},
 		Tags: &compute.Tags{
 			Items: c.Tags,

builder/googlecompute/step_create_instance.go

@@ -117,6 +117,7 @@ func (s *StepCreateInstance) Run(_ context.Context, state multistep.StateBag) mu
 		OnHostMaintenance: c.OnHostMaintenance,
 		Preemptible:       c.Preemptible,
 		Region:            c.Region,
+		ServiceAccountEmail: c.ServiceAccountEmail,
 		Scopes:            c.Scopes,
 		Subnetwork:        c.Subnetwork,
 		Tags:              c.Tags,