Skip unneeded steps (cirruslabs#126)

* Skip run step if we don't have boot commands or communicator

Without a communicator we can't provision, and if we don't have
boot commands there's not any keys to send to the running VM, so
running it is not needed.

This allows configuring builders that only create the VM,
in preparation for later OS installation and provisioning:

source "tart-cli" "create-only" {
  cpu_count    = 4
  memory_gb    = 8
  disk_size_gb = 40
  from_ipsw    = var.ipsw_file
  vm_name      = "pre-install-vm"
  communicator = "none"
}

* Skip typeBootCommandOverVNC if there are no boot commands

There's no need to connect to VNC if we have no boot commands
to type.

builder/tart/builder.go

@@ -104,10 +104,14 @@ func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (pack
 	steps = append(steps,
 		new(stepSetVM),
 		new(stepDiskFilePrepare),
-		new(stepRun),
 	)
 
-	if !b.config.Recovery {
+	communicatorConfigured := b.config.CommunicatorConfig.Type != "none"
+	if len(b.config.BootCommand) > 0 || communicatorConfigured {
+		steps = append(steps, new(stepRun))
+	}
+
+	if !b.config.Recovery && communicatorConfigured {
 		steps = append(steps,
 			&communicator.StepConnect{
 				Config: &b.config.CommunicatorConfig,

builder/tart/step_run.go

@@ -73,7 +73,7 @@ func (s *stepRun) Run(ctx context.Context, state multistep.StateBag) multistep.S
 
 	state.Put("tart-cmd", cmd)
 
-	if (len(config.FromISO) == 0) && !config.DisableVNC {
+	if len(config.BootCommand) > 0 && (len(config.FromISO) == 0) && !config.DisableVNC {
 		if !typeBootCommandOverVNC(ctx, state, config, ui, stdout) {
 			return multistep.ActionHalt
 		}