Allow data lookup on google_active_folder without providing a parent

Google folder lookup API finds folders perfectly fine without providing the folder parent:

```
"query": "lifecycleState=ACTIVE AND displayName=\"Engineering\""

{
  "folders": [
    {
      "name": "folders/<Redacted>",
      "parent": "organizations/<Redacted>",
      "displayName": "Engineering",
      "lifecycleState": "ACTIVE",
      "createTime": "2019-06-14T09:20:35.233Z"
    }
  ]
}

"query": "lifecycleState=ACTIVE AND displayName=\"Global Dev\""

Results returned just fine:
{
  "folders": [
    {
      "name": "folders/<Redacted>",
      "parent": "folders/<Redacted>",
      "displayName": "Global Dev",
      "lifecycleState": "ACTIVE",
      "createTime": "2020-06-16T11:41:45.998Z"
    }
  ]
}
```

google/data_source_google_active_folder.go

@@ -14,7 +14,7 @@ func dataSourceGoogleActiveFolder() *schema.Resource {
 		Schema: map[string]*schema.Schema{
 			"parent": {
 				Type:     schema.TypeString,
-				Required: true,
+				Optional: true,
 			},
 			"display_name": {
 				Type:     schema.TypeString,
@@ -35,10 +35,16 @@ func dataSourceGoogleActiveFolderRead(d *schema.ResourceData, meta interface{})
 		return err
 	}
 
-	parent := d.Get("parent").(string)
 	displayName := d.Get("display_name").(string)
+	queryString := ""
+
+	// parent is optional
+	if parent, ok := d.GetOk("parent"); ok {
+		queryString := fmt.Sprintf("lifecycleState=ACTIVE AND parent=%s AND displayName=\"%s\"", parent.(string), displayName)
+	} else {
+		queryString := fmt.Sprintf("lifecycleState=ACTIVE AND displayName=\"%s\"", displayName)
+	}
 
-	queryString := fmt.Sprintf("lifecycleState=ACTIVE AND parent=%s AND displayName=\"%s\"", parent, displayName)
 	searchRequest := &resourceManagerV2Beta1.SearchFoldersRequest{
 		Query: queryString,
 	}

google/data_source_google_active_folder_test.go

@@ -98,6 +98,26 @@ func testAccDataSourceGoogleActiveFolderCheck(data_source_name string, resource_
 	}
 }
 
+func TestAccDataSourceGoogleActiveFolder_no_parent(t *testing.T) {
+	org := getTestOrgFromEnv(t)
+
+	parent := fmt.Sprintf("organizations/%s", org)
+	displayName := "terraform-test-" + randString(t, 10)
+
+	vcrTest(t, resource.TestCase{
+		PreCheck:  func() { testAccPreCheck(t) },
+		Providers: testAccProviders,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccDataSourceGoogleActiveFolderConfig(parent, displayName),
+				Check: resource.ComposeTestCheckFunc(
+					testAccDataSourceGoogleActiveFolderCheck("data.google_active_folder.my_folder", "google_folder.foobar"),
+				),
+			},
+		},
+	})
+}
+
 func testAccDataSourceGoogleActiveFolderConfig(parent string, displayName string) string {
 	return fmt.Sprintf(`
 resource "google_folder" "foobar" {
@@ -111,3 +131,16 @@ data "google_active_folder" "my_folder" {
 }
 `, parent, displayName)
 }
+
+func testAccDataSourceGoogleActiveFolderConfig_no_parent(parent string, displayName string) string {
+	return fmt.Sprintf(`
+resource "google_folder" "foobar" {
+  parent       = "%s"
+  display_name = "%s"
+}
+
+data "google_active_folder" "my_folder" {
+  display_name = google_folder.foobar.display_name
+}
+`, parent, displayName)
+}

website/docs/d/active_folder.html.markdown

@@ -13,20 +13,38 @@ Get an active folder within GCP by `display_name` and `parent`.
 
 ## Example Usage
 
+To find a folder by its name:
+
+```tf
+data "google_active_folder" "department1" {
+  display_name = "Department 1"
+}
+```
+
+To find a folder in the organization:
+
 ```tf
 data "google_active_folder" "department1" {
   display_name = "Department 1"
   parent       = "organizations/1234567"
 }
 ```
+To find a folder nested in another folder:
+
+```tf
+data "google_active_folder" "department1" {
+  display_name = "Department 1"
+  parent       = "folders/12345"
+}
+```
 
 ## Argument Reference
 
 The following arguments are supported:
 
 * `display_name` - (Required) The folder's display name.
 
-* `parent` - (Required) The resource name of the parent Folder or Organization.
+* `parent` - (Optional) The resource id of the parent Folder or Organization.
 
 ## Attributes Reference