tests/resource/aws_cloudtrail: Fix syntax for Terraform 0.12
tests/resource/aws_cloudtrail: Fix syntax for Terraform 0.12

This change is backwards compatible with Terraform 0.11.

Previous output from Terraform 0.12 acceptance testing:

```
--- FAIL: TestAccAWSCloudTrail (209.69s)
    --- FAIL: TestAccAWSCloudTrail/Trail (209.69s)
        --- FAIL: TestAccAWSCloudTrail/Trail/tags (0.51s)
            testing.go:568: Step 0 error: config is invalid: Unsupported block type: Blocks of type "tags" are not expected here. Did you mean to define argument "tags"? If so, use the equals sign to assign it a value.
```

Output from Terraform 0.12 acceptance testing:

```
        --- PASS: TestAccAWSCloudTrail/Trail/tags (93.14s)
```

Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws