see https://groups.google.com/forum/#!topic/packer-tool/LYZ5oesLbEQ

base.json

{
    "builders": [
        {
            "type": "virtualbox",
            "iso_url": "http://cdimage.debian.org/debian-cd/7.1.0/i386/iso-cd/debian-7.1.0-i386-CD-1.iso",
            "iso_md5": "c29ec2db6754dadd341b9763164aca9a",
            "ssh_username": "packer",
            "ssh_password": "packer",
            "guest_os_type": "Debian",
            "http_directory": ".",
            "boot_wait": "5s",
            "boot_command": ["<esc><wait>auto url=http://{{ .HTTPIP }}:{{ .HTTPPort }}/preseed.cfg<enter>"]
        }
    ]
}

ci.json

{
    "includes": [
        "relative/path/to/base.json",
    ],

    "provisioners": [
        {
            "type": "shell",
            "script": "provision/ci.bash",
            "execute_command": "{{ .Vars }} sudo bash {{ .Path }}"
        }
    ]
}

web.json

{
    "includes": [
        "relative/path/to/base.json",
        "/absolute/path/to/base.json",
        "http://over.http.too/base.json",
        "[email protected]:you/get/the/point/base.json"
    ],

    "provisioners": [
        {
            "type": "shell",
            "script": "provision/web.bash",
            "execute_command": "{{ .Vars }} sudo bash {{ .Path }}"
        }
    ]
}