The Mac/Bento templates use Tim Sutton's amazingly awesome os-vm-templates project to prepare a .dmg that can perform an unattended install of Mac OS X 10.7 through to 10.11. You'll need a copy of the Mac OS X .app on locally on disk, so we'll assume that it's installed under /Applications from the App Store.
Clone the os-vm-templates project and run the prepare_iso.sh program to produce the disk image.
git clone https://github.com/timsutton/osx-vm-templates.git
cd osx-vm-templates
sudo prepare_iso/prepare_iso.sh "/Applications/Install OS X El Capitan.app" out