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/ubuntu-16.04/template.json b/ubuntu-16.04/template.json
index aa80d0a..fc05680 100644
--- a/ubuntu-16.04/template.json
+++ b/ubuntu-16.04/template.json
@@ -234,7 +234,7 @@
       "format": "qcow2",
       "iso_checksum": "566efef1d6f12e7d3a994c2405bdb642",
       "iso_checksum_type": "md5",
-      "iso_url": "https://cloud-images.ubuntu.com/releases/16.04/release-20180306/ubuntu-16.04-server-cloudimg-amd64-disk1.img",
+      "iso_url": "http://cloud-images.ubuntu.com/releases/xenial/release-20180306/ubuntu-16.04-server-cloudimg-amd64-disk1.img",
       "disk_image": true,
       "accelerator": "kvm",
       "disk_size": "{{ user `disk_size`}}",