Update mirror|ubuntu templatest

  * Use latest and more strict qcow2 path
  * Be polite with pre-cloned repos in mirror
  * Use correct checkout for tag checkout

Change-Id: I311e4f4e8be0d451cdcb9568cfc8b9baaf525db7
diff --git a/mirror-image/template.json b/mirror-image/template.json
index f606787..1f12c3a 100644
--- a/mirror-image/template.json
+++ b/mirror-image/template.json
@@ -145,11 +145,11 @@
       "format": "qcow2",
       "accelerator": "kvm",
       "disk_size": "{{ user `disk_size`}}",
-      "iso_url": "http://cloud-images.ubuntu.com/xenial/20180224/xenial-server-cloudimg-amd64-disk1.img",
+      "iso_url": "http://cloud-images.ubuntu.com/releases/xenial/release-20180306/ubuntu-16.04-server-cloudimg-amd64-disk1.img",
       "iso_target_path": "/root/isos/xenial-server-cloudimg-amd64-disk1.img",
       "iso_checksum_type": "md5",
       "disk_image": true,
-      "iso_checksum": "697d8f9058ad118cea699d14ae414f8b",
+      "iso_checksum": "566efef1d6f12e7d3a994c2405bdb642",
       "http_directory": "http",
       "headless": true,
       "ssh_username": "{{user `user`}}",