Update the ubuntu regex

Problem:
amazon-ebs: No AMI was found matching filters: {
   Filters: [{
       Name: "name",
       Values: ["*Ubuntu*16.04*"]
     },{
       Name: "root-device-type",
       Values: ["ebs"]
     },{
       Name: "virtualization-type",
       Values: ["hvm"]
     }],
   Owners: ["099720109477"]
}

Change-Id: I46b54bda7fafade1367624261363c22ab1b4d5fa
diff --git a/packer/vars/ubuntu-16.04.json b/packer/vars/ubuntu-16.04.json
index 370af61..94b2014 100644
--- a/packer/vars/ubuntu-16.04.json
+++ b/packer/vars/ubuntu-16.04.json
@@ -1,5 +1,5 @@
 {
-  "source_ami_filter_name": "*Ubuntu*16.04*",
+  "source_ami_filter_name": "*ubuntu*16.04*",
   "source_ami_filter_owner": "099720109477",
   "cloud_user": "ubuntu",