supported `preemptible` in googlecompute builder

builder/googlecompute/config.go

@@ -31,6 +31,7 @@ type Config struct {
 	MachineType          string            `mapstructure:"machine_type"`
 	Metadata             map[string]string `mapstructure:"metadata"`
 	Network              string            `mapstructure:"network"`
+	Preemptible          bool              `mapstructure:"preemptible"`
 	SourceImage          string            `mapstructure:"source_image"`
 	SourceImageProjectId string            `mapstructure:"source_image_project_id"`
 	RawStateTimeout      string            `mapstructure:"state_timeout"`

builder/googlecompute/driver.go

@@ -47,6 +47,7 @@ type InstanceConfig struct {
 	Metadata    map[string]string
 	Name        string
 	Network     string
+	Preemptible bool
 	Tags        []string
 	Zone        string
 }

builder/googlecompute/driver_gce.go

@@ -255,6 +255,9 @@ func (d *driverGCE) RunInstance(c *InstanceConfig) (<-chan error, error) {
 				Network: network.SelfLink,
 			},
 		},
+		Scheduling: &compute.Scheduling{
+			Preemptible: c.Preemptible,
+		},
 		ServiceAccounts: []*compute.ServiceAccount{
 			&compute.ServiceAccount{
 				Email: "default",

builder/googlecompute/step_create_instance.go

@@ -59,6 +59,7 @@ func (s *StepCreateInstance) Run(state multistep.StateBag) multistep.StepAction
 		Metadata:    config.getInstanceMetadata(sshPublicKey),
 		Name:        name,
 		Network:     config.Network,
+		Preemptible: config.Preemptible,
 		Tags:        config.Tags,
 		Zone:        config.Zone,
 	})