Fix basebuild job by adding multiarch support (was trying to build on x86_64 with an arm64 image)

Change-Id: Ie9be1a1a7aac016139e17d3293e22e744d527eaa
diff --git a/packer/vars/ubuntu-16.04.json b/packer/vars/ubuntu-16.04.json
index b0e9132..972ff75 100644
--- a/packer/vars/ubuntu-16.04.json
+++ b/packer/vars/ubuntu-16.04.json
@@ -1,8 +1,10 @@
 {
   "source_ami_filter_name": "*ubuntu*16.04*",
   "source_ami_filter_owner": "099720109477",
+
   "ssh_user": "ubuntu",
 
   "distro": "Ubuntu 16.04",
+  "arch": "x86_64",
   "cloud_user_data": "provision/null_data.sh"
 }