my packer tempalte

gistfile1.txt

{
  "builders": [
    {
      "type": "virtualbox-iso",
      "name": "mybox",
      "guest_os_type": "Ubuntu_64",
      "iso_url": "file:///home/stavros/Downloads/ubuntu-16.04-server-amd64.iso",
      "iso_checksum": "23e97cd5d4145d4105fbf29878534049",
      "iso_checksum_type": "md5",
      "ssh_username": "user",
      "ssh_password": "user",
      "ssh_port": 22,
      "ssh_wait_timeout": "10000s",
      "shutdown_command": "echo 'packer' | sudo -S shutdown -P now",
      "boot_wait": "10s",
      "guest_additions_mode": "upload",
      "guest_additions_path" : "VBoxGuestAdditions_{{.Version}}.iso",
      "headless": false,
      "http_directory" : "cfg",
      "boot_command" : [
        "<esc><esc><enter><wait>",
        "/install/vmlinuz noapic auto=true preseed/url=http://{{ .HTTPIP }}:{{ .HTTPPort }}/preseed.cfg <wait>",
        "debian-installer=en_US auto locale=en_US kbd-chooser/method=us <wait>",
        "hostname={{ .Name }} <wait>",
        "fb=false debconf/frontend=noninteractive <wait>",
        "keyboard-configuration/modelcode=SKIP keyboard-configuration/layout=USA keyboard-configuration/variant=USA console-setup/ask_detect=false <wait>",
        "initrd=/install/initrd.gz -- <enter><wait>"
      ],
      "vboxmanage": [
        [
          "modifyvm",
          "{{.Name}}",
          "--memory",
          "2048"
        ],
        [
          "modifyvm",
          "{{.Name}}",
          "--cpus",
          "4"
        ],
        [
          "modifyvm",
          "{{.Name}}",
          "--vram",
          "64"
        ]
      ]
    }
  ],

  "provisioners": [
    {
      "type": "shell",
      "script": "install.sh",
      "execute_command": "echo 'user' | {{ .Vars }} sudo -E -S sh '{{ .Path }}'"
    }
  ]
}

My preseed file is under cfg folder.
Using either
https://help.ubuntu.com/14.04/installation-guide/example-preseed.txt
or
https://help.ubuntu.com/lts/installation-guide/example-preseed.txt