Add error if boot disk specified on non-first disk in instance template

.changelog/2996.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+compute: `google_compute_instance_template` add plan time check for any disks marked `boot` outside of the first disk
+```

google/resource_compute_instance_template.go

@@ -41,6 +41,7 @@ func resourceComputeInstanceTemplate() *schema.Resource {
 		CustomizeDiff: customdiff.All(
 			resourceComputeInstanceTemplateSourceImageCustomizeDiff,
 			resourceComputeInstanceTemplateScratchDiskCustomizeDiff,
+			resourceComputeInstanceTemplateBootDiskCustomizeDiff,
 		),
 		MigrateState: resourceComputeInstanceTemplateMigrateState,
 
@@ -577,6 +578,20 @@ func resourceComputeInstanceTemplateScratchDiskCustomizeDiffFunc(diff TerraformR
 	return nil
 }
 
+func resourceComputeInstanceTemplateBootDiskCustomizeDiff(diff *schema.ResourceDiff, meta interface{}) error {
+	numDisks := diff.Get("disk.#").(int)
+	// No disk except the first can be the boot disk
+	for i := 1; i < numDisks; i++ {
+		key := fmt.Sprintf("disk.%d.boot", i)
+		if v, ok := diff.GetOk(key); ok {
+			if v.(bool) {
+				return fmt.Errorf("Only the first disk specified in instance_template can be the boot disk. %s was true", key)
+			}
+		}
+	}
+	return nil
+}
+
 func buildDisks(d *schema.ResourceData, config *Config) ([]*computeBeta.AttachedDisk, error) {
 	project, err := getProject(d, config)
 	if err != nil {

google/resource_sql_database_instance_test.go

@@ -10,7 +10,6 @@ import (
 	"github.com/hashicorp/terraform-plugin-sdk/helper/acctest"
 	"github.com/hashicorp/terraform-plugin-sdk/helper/resource"
 	"github.com/hashicorp/terraform-plugin-sdk/terraform"
-
 	sqladmin "google.golang.org/api/sqladmin/v1beta4"
 )