skip guest accelerators if count is 0.

Instances in instance groups in google will fail to provision, despite
requesting 0 GPUs. This came up for me when trying to provision
a similar instance group in all available regions, but only asking for
GPU's in those that support them by parameterizing the `count` and
setting it to 0.

This might be a violation of some terraform principles. For example,
testing locally with this change `terraform` did not recognize that
indeed my infra needed to be re-deployed (from it's pov, I assume it
believes this because inputs hadn't changed). Additionally, there may be
valid reasons for creating an instance template with 0 gpu's that can be
tuned upwards.

google/resource_compute_instance.go

@@ -1198,6 +1198,9 @@ func expandInstanceGuestAccelerators(d TerraformResourceData, config *Config) ([
 	guestAccelerators := make([]*computeBeta.AcceleratorConfig, len(accels))
 	for i, raw := range accels {
 		data := raw.(map[string]interface{})
+		if data["count"].(int) == 0 {
+			continue
+		}
 		at, err := ParseAcceleratorFieldValue(data["type"].(string), d, config)
 		if err != nil {
 			return nil, fmt.Errorf("cannot parse accelerator type: %v", err)

google/resource_compute_instance_template.go

@@ -506,6 +506,9 @@ func expandInstanceTemplateGuestAccelerators(d TerraformResourceData, config *Co
 	guestAccelerators := make([]*computeBeta.AcceleratorConfig, len(accels))
 	for i, raw := range accels {
 		data := raw.(map[string]interface{})
+		if data["count"].(int) == 0 {
+			continue
+		}
 		guestAccelerators[i] = &computeBeta.AcceleratorConfig{
 			AcceleratorCount: int64(data["count"].(int)),
 			// We can't use ParseAcceleratorFieldValue here because an instance

google/resource_compute_instance_template_test.go

@@ -301,7 +301,7 @@ func TestAccComputeInstanceTemplate_guestAccelerator(t *testing.T) {
 		CheckDestroy: testAccCheckComputeInstanceTemplateDestroy,
 		Steps: []resource.TestStep{
 			resource.TestStep{
-				Config: testAccComputeInstanceTemplate_guestAccelerator(acctest.RandString(10)),
+				Config: testAccComputeInstanceTemplate_guestAccelerator(acctest.RandString(10), 1),
 				Check: resource.ComposeTestCheckFunc(
 					testAccCheckComputeInstanceTemplateExists("google_compute_instance_template.foobar", &instanceTemplate),
 					testAccCheckComputeInstanceTemplateHasGuestAccelerator(&instanceTemplate, "nvidia-tesla-k80", 1),
@@ -312,6 +312,28 @@ func TestAccComputeInstanceTemplate_guestAccelerator(t *testing.T) {
 
 }
 
+func TestAccComputeInstanceTemplate_guestAcceleratorSkip(t *testing.T) {
+	t.Parallel()
+
+	var instanceTemplate compute.InstanceTemplate
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckComputeInstanceTemplateDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: testAccComputeInstanceTemplate_guestAccelerator(acctest.RandString(10), 0),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckComputeInstanceTemplateExists("google_compute_instance_template.foobar", &instanceTemplate),
+					testAccCheckComputeInstanceTemplateLacksGuestAccelerator(&instanceTemplate),
+				),
+			},
+		},
+	})
+
+}
+
 func TestAccComputeInstanceTemplate_minCpuPlatform(t *testing.T) {
 	t.Parallel()
 
@@ -604,6 +626,16 @@ func testAccCheckComputeInstanceTemplateHasGuestAccelerator(instanceTemplate *co
 	}
 }
 
+func testAccCheckComputeInstanceTemplateLacksGuestAccelerator(instanceTemplate *compute.InstanceTemplate) resource.TestCheckFunc {
+	return func(s *terraform.State) error {
+		if len(instanceTemplate.Properties.GuestAccelerators) > 0 {
+			return fmt.Errorf("Expected no guest accelerators")
+		}
+
+		return nil
+	}
+}
+
 func testAccCheckComputeInstanceTemplateHasMinCpuPlatform(instanceTemplate *compute.InstanceTemplate, minCpuPlatform string) resource.TestCheckFunc {
 	return func(s *terraform.State) error {
 		if instanceTemplate.Properties.MinCpuPlatform != minCpuPlatform {
@@ -1027,7 +1059,7 @@ resource "google_compute_instance_template" "foobar" {
 }`, i, i, i)
 }
 
-func testAccComputeInstanceTemplate_guestAccelerator(i string) string {
+func testAccComputeInstanceTemplate_guestAccelerator(i string, count int) string {
 	return fmt.Sprintf(`
 resource "google_compute_instance_template" "foobar" {
 	name = "instance-test-%s"
@@ -1050,10 +1082,10 @@ resource "google_compute_instance_template" "foobar" {
 	}
 
 	guest_accelerator {
-		count = 1
+		count = %d
 		type = "nvidia-tesla-k80"
 	}
-}`, i)
+}`, i, count)
 }
 
 func testAccComputeInstanceTemplate_minCpuPlatform(i string) string {