restructure JSON terraform config block AST

When configuration is read out of JSON, HCL assumes that empty levels of
objects can be flattened, but this removes too much to decode into a
config.Terraform struct.

Reconstruct the appropriate AST to decode the config struct.

config/loader_hcl.go

@@ -209,6 +209,19 @@ func loadTerraformHcl(list *ast.ObjectList) (*Terraform, error) {
 	// Get our one item
 	item := list.Items[0]
 
+	// This block should have an empty top level ObjectItem.  If there are keys
+	// here, it's likely because we have a flattened JSON object, and we can
+	// lift this into a nested ObjectList to decode properly.
+	if len(item.Keys) > 0 {
+		item = &ast.ObjectItem{
+			Val: &ast.ObjectType{
+				List: &ast.ObjectList{
+					Items: []*ast.ObjectItem{item},
+				},
+			},
+		}
+	}
+
 	// We need the item value as an ObjectList
 	var listVal *ast.ObjectList
 	if ot, ok := item.Val.(*ast.ObjectType); ok {

config/loader_test.go

@@ -384,6 +384,32 @@ backend (s3)
 	}
 }
 
+// test that the alternate, more obvious JSON format also decodes properly
+func TestLoadFile_terraformBackendJSON2(t *testing.T) {
+	c, err := LoadFile(filepath.Join(fixtureDir, "terraform-backend-2.tf.json"))
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if c == nil {
+		t.Fatal("config should not be nil")
+	}
+
+	if c.Dir != "" {
+		t.Fatalf("bad: %#v", c.Dir)
+	}
+
+	{
+		actual := terraformStr(c.Terraform)
+		expected := strings.TrimSpace(`
+backend (s3)
+  foo`)
+		if actual != expected {
+			t.Fatalf("bad:\n%s", actual)
+		}
+	}
+}
+
 func TestLoadFile_terraformBackendMulti(t *testing.T) {
 	_, err := LoadFile(filepath.Join(fixtureDir, "terraform-backend-multi.tf"))
 	if err == nil {

config/test-fixtures/terraform-backend-2.tf.json

@@ -0,0 +1,9 @@
+{
+  "terraform": {
+    "backend": {
+      "s3": {
+        "foo": "bar"
+      }
+    }
+  }
+}