Support exclusions (notScopes) in azurerm_policy_assignment

[olohmann]

This PR adds support for exlcusions (notScopes) in Azure Policy Assignments (resource azurerm_policy_set_definition).

Addresses issue #1138

Test Results:

> $ make testacc TESTARGS='-run=TestAccAzureRMPolicyAssignment'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccAzureRMPolicyAssignment -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   TestAccAzureRMPolicyAssignment_basic
=== PAUSE TestAccAzureRMPolicyAssignment_basic
=== RUN   TestAccAzureRMPolicyAssignment_deployIfNotExists_policy
=== PAUSE TestAccAzureRMPolicyAssignment_deployIfNotExists_policy
=== RUN   TestAccAzureRMPolicyAssignment_complete
=== PAUSE TestAccAzureRMPolicyAssignment_complete
=== RUN   TestAccAzureRMPolicyAssignment_not_scopes
=== PAUSE TestAccAzureRMPolicyAssignment_not_scopes
=== CONT  TestAccAzureRMPolicyAssignment_basic
=== CONT  TestAccAzureRMPolicyAssignment_not_scopes
=== CONT  TestAccAzureRMPolicyAssignment_complete
=== CONT  TestAccAzureRMPolicyAssignment_deployIfNotExists_policy
--- PASS: TestAccAzureRMPolicyAssignment_basic (247.49s)
--- PASS: TestAccAzureRMPolicyAssignment_complete (247.53s)
--- PASS: TestAccAzureRMPolicyAssignment_not_scopes (249.56s)
--- PASS: TestAccAzureRMPolicyAssignment_deployIfNotExists_policy (250.12s)

(fixed #1138)

[katbyte]

@olohmann,

Thank you for the PR, aside from one minor comment i've left in this LGTM 🚀 Hope you don't mind but i am going to fix it now so i can merge 🙂

[katbyte]

We can just leave this out if ForceNew is false.

Suggested change

ForceNew: false,

[ghost]

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