provider/azurerm: `azurerm_dns_zone` now returns `name_servers` (#7434)
provider/azurerm: `azurerm_dns_zone` now returns `name_servers` (#7434)

Fixes #7374

The introduction of the AzureRM SDK 3.0.0-beta means that the
`name_servers` for the DNS Zone are returned from the API

This PR has a dependency on #7420 being merged first

```
make testacc TEST=./builtin/providers/azurerm TESTARGS='-run=TestAccAzureRMDnsZone_'
==> Checking that code complies with gofmt requirements...
/Users/stacko/Code/go/bin/stringer
go generate $(go list ./... | grep -v /vendor/)
2016/06/30 15:20:01 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/azurerm -v
-run=TestAccAzureRMDnsZone_ -timeout 120m
=== RUN   TestAccAzureRMDnsZone_basic
--- PASS: TestAccAzureRMDnsZone_basic (92.42s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/azurerm
92.444s
```
sebastus/terraform-provider-azurerm,thiagocaiubi/terraform-provider-azurerm,abhijeetgaiha/terraform-provider-azurerm,abhijeetgaiha/terraform-provider-azurerm,abhijeetgaiha/terraform-provider-azurerm,thiagocaiubi/terraform-provider-azurerm,sebastus/terraform-provider-azurerm,sebastus/terraform-provider-azurerm,thiagocaiubi/terraform-provider-azurerm