Fixed parameter name for policy_arn setting

[mvisonneau]

This solves an issue that prevents the use of policy_arn setting on the vault_aws_secret_backend_role resource.

eg:

module.vault_aws.vault_aws_secret_backend_role.admin: Creating...
  backend:    "" => "aws"
  name:       "" => "admin"
  policy_arn: "" => "arn:aws:iam::aws:policy/AdministratorAccess"

Error: Error applying plan:

1 error(s) occurred:

* module.vault_aws.vault_aws_secret_backend_role.admin: 1 error(s) occurred:

* vault_aws_secret_backend_role.admin: Error creating role "admin" for backend "aws": Error making API request.

URL: PUT https://vault.example.com/v1/aws/roles/admin
Code: 500. Errors:

* 1 error occurred:

* Either policy or arn must be provided

[paulRbr]

These changes seem pretty straight forward. Especially when seeing the vault API documentation for the endpoint: https://www.vaultproject.io/api/secret/aws/index.html#create-update-role

LGTM.

[paddycarver]

Hi @mvisonneau, thanks so much for the PR! The change looks great. Do you mind, however, just adding a test case that exercises the broken code? If you don't have time or, understandably, have moved on while waiting for a reply to this, I'll come back at the end of the week and add the test case and push this forward. Thanks again!

[mvisonneau]

Thanks for the feedback @paddycarver, I've added the tests.

[paddycarver]

I think I'd prefer if this had been a new test function, instead of getting added to an existing function, but honestly that's more a stylistic concern, and I'd like to get this fix rolled out. Thanks so much for taking the time to PR this!

[mvisonneau]

thanks! :) will do next time