Adding a test case for a role using DataActions

Tests pass:

```
$ acctests azurerm TestAccAzureRMRole
=== RUN   TestAccAzureRMRoleDefinition_importBasic
--- PASS: TestAccAzureRMRoleDefinition_importBasic (24.71s)
=== RUN   TestAccAzureRMRoleDefinition_importComplete
--- PASS: TestAccAzureRMRoleDefinition_importComplete (14.30s)
=== RUN   TestAccAzureRMRoleAssignment
=== RUN   TestAccAzureRMRoleAssignment/basic
=== RUN   TestAccAzureRMRoleAssignment/basic/emptyName
=== RUN   TestAccAzureRMRoleAssignment/basic/roleName
=== RUN   TestAccAzureRMRoleAssignment/basic/dataActions
=== RUN   TestAccAzureRMRoleAssignment/basic/builtin
=== RUN   TestAccAzureRMRoleAssignment/basic/custom
=== RUN   TestAccAzureRMRoleAssignment/import
=== RUN   TestAccAzureRMRoleAssignment/import/basic
=== RUN   TestAccAzureRMRoleAssignment/import/custom
--- PASS: TestAccAzureRMRoleAssignment (140.30s)
    --- PASS: TestAccAzureRMRoleAssignment/basic (95.32s)
        --- PASS: TestAccAzureRMRoleAssignment/basic/emptyName (19.34s)
        --- PASS: TestAccAzureRMRoleAssignment/basic/roleName (15.95s)
        --- PASS: TestAccAzureRMRoleAssignment/basic/dataActions (19.54s)
        --- PASS: TestAccAzureRMRoleAssignment/basic/builtin (17.11s)
        --- PASS: TestAccAzureRMRoleAssignment/basic/custom (23.38s)
    --- PASS: TestAccAzureRMRoleAssignment/import (44.98s)
        --- PASS: TestAccAzureRMRoleAssignment/import/basic (22.07s)
        --- PASS: TestAccAzureRMRoleAssignment/import/custom (22.91s)
=== RUN   TestAccAzureRMRoleDefinition_basic
--- PASS: TestAccAzureRMRoleDefinition_basic (13.78s)
=== RUN   TestAccAzureRMRoleDefinition_complete
--- PASS: TestAccAzureRMRoleDefinition_complete (12.98s)
=== RUN   TestAccAzureRMRoleDefinition_update
--- PASS: TestAccAzureRMRoleDefinition_update (23.25s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	229.356s
```

azurerm/resource_arm_role_assignment_test.go

@@ -16,10 +16,11 @@ func TestAccAzureRMRoleAssignment(t *testing.T) {
 	// Azure only being happy about provisioning a couple at a time
 	testCases := map[string]map[string]func(t *testing.T){
 		"basic": {
-			"emptyName": testAccAzureRMRoleAssignment_emptyName,
-			"roleName":  testAccAzureRMRoleAssignment_roleName,
-			"builtin":   testAccAzureRMRoleAssignment_builtin,
-			"custom":    testAccAzureRMRoleAssignment_custom,
+			"emptyName":   testAccAzureRMRoleAssignment_emptyName,
+			"roleName":    testAccAzureRMRoleAssignment_roleName,
+			"dataActions": testAccAzureRMRoleAssignment_dataActions,
+			"builtin":     testAccAzureRMRoleAssignment_builtin,
+			"custom":      testAccAzureRMRoleAssignment_custom,
 		},
 		"import": {
 			"basic":  testAccAzureRMRoleAssignment_importBasic,
@@ -81,6 +82,27 @@ func testAccAzureRMRoleAssignment_roleName(t *testing.T) {
 	})
 }
 
+func testAccAzureRMRoleAssignment_dataActions(t *testing.T) {
+	id := uuid.New().String()
+	resourceName := "azurerm_role_assignment.test"
+	config := testAccAzureRMRoleAssignment_dataActionsConfig(id)
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testCheckAzureRMRoleAssignmentDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: config,
+				Check: resource.ComposeTestCheckFunc(
+					testCheckAzureRMRoleAssignmentExists(resourceName),
+					resource.TestCheckResourceAttrSet(resourceName, "role_definition_id"),
+				),
+			},
+		},
+	})
+}
+
 func testAccAzureRMRoleAssignment_builtin(t *testing.T) {
 	id := uuid.New().String()
 	config := testAccAzureRMRoleAssignment_builtinConfig(id)
@@ -206,6 +228,21 @@ resource "azurerm_role_assignment" "test" {
 `, id)
 }
 
+func testAccAzureRMRoleAssignment_dataActionsConfig(id string) string {
+	return fmt.Sprintf(`
+data "azurerm_subscription" "primary" {}
+
+data "azurerm_client_config" "test" {}
+
+resource "azurerm_role_assignment" "test" {
+  name                 = "%s"
+  scope                = "${data.azurerm_subscription.primary.id}"
+  role_definition_name = "Virtual Machine User Login"
+  principal_id         = "${data.azurerm_client_config.test.service_principal_object_id}"
+}
+`, id)
+}
+
 func testAccAzureRMRoleAssignment_builtinConfig(id string) string {
 	return fmt.Sprintf(`
 data "azurerm_subscription" "primary" {}