Error in ouputs sensitive values with terraform v0.15.0

Hi, with terraform `v0.15.0` it give the next error:
```
Error: Output refers to sensitive values
│
│ on .terraform/modules/documentdb_cluster/outputs.tf line 1:
│ 1: output "master_username" {
│
│ Expressions used in outputs can only refer to sensitive values if the sensitive attribute is true.
```
module version: `0.13.0`

Thanks!

Inspect .terraform/modules/documentdb_cluster/outputs.tf at line 1 and reproduce the output validation error with Terraform v0.15.0 and module version 0.13.0. Determine how the master_username output should declare its sensitive status, then verify that Terraform accepts the module outputs without the reported error.

Written by the indexing model from the issue text.