Fix the code logic to generate the required provider list.

[metacpp]

This PR is created to address the regression issue after upgrading to v0.4.0 recently:
hashicorp/terraform-provider-azurerm#3312

Before making this change, if you ran any acceptance test like TestAccAzureRMResourceGroup_basic, you will see below error:

Error: Error ensuring Resource Providers are registered: Cannot register provider Microsoft.Cdn 
with Azure Resource Manager: resources.ProvidersClient#Register: Failure responding to request:
StatusCode=404 -- Original Error: autorest/azure: Service returned an error. Status=404 
Code="InvalidResourceNamespace" Message="The resource namespace 'Microsoft.Cdn' is invalid.".

With this change, the test can pass.

[hashicorp-cla]

All committers have signed the CLA.

[katbyte]

Thanks for this @metacpp! LGTM