vault_aws_sts: correct terraform_backend_usage

Create new vault_aws_secret_backend_role terraform_backend_usage to use S3/DDB from remote_state.

Applied on infra-stg for validation.

How to test:

# GET AWS creds
VAULT_ADDR=https://vault.infra-stg.caascad.com/ vault read aws/sts/terraform_backend_usage

# Then export
export AWS_ACCESS_KEY_ID=XXX
export AWS_SECRET_KEY=XXX
export AWS_SESSION_TOKEN=XXX

tmp_dir=$(mktemp -d)

cd ${tmp_dir}
cat <<EOF > backend.ini
bucket = "infra-stg-181151069204-tf-states"
region = "eu-west-3"
dynamodb_table = "infra-stg-181151069204-tf-locks"
EOF

cat <<EOF > main.tf
terraform {
  backend "s3" {
    key = "mvpkaas-338/test/terraform.tfstate"
  }
}
resource "time_static" "example" {}

output "current_time" {
  value = time_static.example.rfc3339
}
EOF

terraform init -backend-config=backend.ini
terraform apply

cd ~-
rm -rf ${tmp_dir}