install a kernel by default

It's kind of interesting that, by default, this module doesn't install a kernel in the extra packages list. I think I know where this is coming from: this was originally deployed on Xen backends, which need a special kernel (or none at all!)... but now most people use KVM as a backend, and that works fine with "vanillay" debian kernels. The README interestingly states this as an example:

  class { ganeti :
    debootstrap_defaults => {
      extra_packages => ['linux-image-amd64', 'dbus', 'libpam-systemd'],
    }
  }

... maybe we should push that further and make it the default? Or clarify why it isn't in the README more strongly?