provider/azurerm: add client_config data source
provider/azurerm: add client_config data source

azurerm_client_config provides access to:

 - client_id
 - tenant_id
 - subscription_id

```
TF_ACC=1 go test ./builtin/providers/azurerm -v -run TestAccAzureRMClientConfig -timeout 120m
=== RUN   TestAccAzureRMClientConfig_basic
--- PASS: TestAccAzureRMClientConfig_basic (27.79s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/azurerm	27.862s
```

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