Update ubuntu 16.04 image link and checksum

Change-Id: Ibca950562b71a0e0466c3489e3c8e8735742adcd
diff --git a/ubuntu-16.04/template.json b/ubuntu-16.04/template.json
index aec11c2..62cf7d9 100644
--- a/ubuntu-16.04/template.json
+++ b/ubuntu-16.04/template.json
@@ -126,9 +126,9 @@
       "output_directory": "images/{{ user `distro` }}-vbox-{{ isotime \"200601021504\"  }}",
       "guest_os_type": "Ubuntu_64",
       "http_directory": "http",
-      "iso_checksum": "2bce60d18248df9980612619ff0b34e6",
+      "iso_checksum": "10fcd20619dce11fe094e960c85ba4a9",
       "iso_checksum_type": "md5",
-      "iso_url": "http://releases.ubuntu.com/16.04/ubuntu-16.04.2-server-amd64.iso",
+      "iso_url": "http://releases.ubuntu.com/16.04/ubuntu-16.04.3-server-amd64.iso",
       "ssh_username": "{{user `user`}}",
       "ssh_password": "{{user `password`}}",
       "ssh_port": 22,