Add preseed to disable signed grub and shim installation

[EOLE team]

Hello,

To work around https://bugs.launchpad.net/ubuntu/+source/grub2/+bug/1625200 we would like to disable the installation of grub-efi-amd64-signed and shim-signed.

The signed grub is installed as soon as we are on EFI 64bits (I did not find the sources for Xenial but here is something similar for Wily https://bazaar.launchpad.net/~ubuntu-branches/ubuntu/wily/grub-installer/wily-proposed/view/head:/grub-installer#L443)

I think a preseed could be added to disable the automatic selection of the signed version.

Regards.