Support for Management Groups in azurerm_policy_set_definition

[olohmann]

This PR adds support for a Management Groups in Azure Policy Set Definitions (resource azurerm_policy_set_definition).

Addresses issue #2617.

Test Results:

> $ make testacc TESTARGS='-run=TestAccAzureRMPolicySetDefinition'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccAzureRMPolicySetDefinition -timeout 180m -ldflags="-X=github.com/terraform-providers/terraform-provider-azurerm/version.ProviderVersion=acc"
?   	github.com/terraform-providers/terraform-provider-azurerm	[no test files]
=== RUN   TestAccAzureRMPolicySetDefinition_builtIn
=== PAUSE TestAccAzureRMPolicySetDefinition_builtIn
=== RUN   TestAccAzureRMPolicySetDefinition_custom
=== PAUSE TestAccAzureRMPolicySetDefinition_custom
=== RUN   TestAccAzureRMPolicySetDefinition_ManagementGroup
=== PAUSE TestAccAzureRMPolicySetDefinition_ManagementGroup
=== CONT  TestAccAzureRMPolicySetDefinition_builtIn
=== CONT  TestAccAzureRMPolicySetDefinition_ManagementGroup
=== CONT  TestAccAzureRMPolicySetDefinition_custom
--- PASS: TestAccAzureRMPolicySetDefinition_builtIn (110.35s)
--- PASS: TestAccAzureRMPolicySetDefinition_ManagementGroup (149.36s)
--- PASS: TestAccAzureRMPolicySetDefinition_custom (202.85s)

(fixed #2617)

[katbyte]

Thank you for the enhancement resource @olohmann! This LGTM 🙂

[ghost]

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