provider/azurerm: expose default keys for servicebus_namespace
provider/azurerm: expose default keys for servicebus_namespace

A default authorization rule is created by Azure which, if present is exported by
the resource.

TF_ACC=1 go test ./builtin/providers/azurerm -v -run TestAccAzureRMServiceBusNamespace -timeout 120m
=== RUN   TestAccAzureRMServiceBusNamespaceCapacity_validation
--- PASS: TestAccAzureRMServiceBusNamespaceCapacity_validation (0.00s)
=== RUN   TestAccAzureRMServiceBusNamespaceSku_validation
--- PASS: TestAccAzureRMServiceBusNamespaceSku_validation (0.00s)
=== RUN   TestAccAzureRMServiceBusNamespace_basic
--- PASS: TestAccAzureRMServiceBusNamespace_basic (352.03s)
=== RUN   TestAccAzureRMServiceBusNamespace_readDefaultKeys
--- PASS: TestAccAzureRMServiceBusNamespace_readDefaultKeys (349.17s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/azurerm	701.278s

thiagocaiubi/terraform-provider-azurerm,sebastus/terraform-provider-azurerm,sebastus/terraform-provider-azurerm,thiagocaiubi/terraform-provider-azurerm,abhijeetgaiha/terraform-provider-azurerm,abhijeetgaiha/terraform-provider-azurerm,sebastus/terraform-provider-azurerm,thiagocaiubi/terraform-provider-azurerm,abhijeetgaiha/terraform-provider-azurerm