Bug#873604: cloud.debian.org: grub shouldn't assume gfxterm

[Mariusz Mazur <mmazur@axeos.com>]

Package: cloud.debian.org

Debian8 openstack image uses extlinux. Debian9 switched to grub.
Unfortunately the default config for grub uses 'terminal_output
gfxterm'. While this runs fine on my openstack setup, it does not boot
at all on a simple libvirt+qemu system I have which utilizes rather
barebones libvirt guest configs. (guest uses 100% cpu, guest kernel
never starts booting)

Unless there's a good reason for using gfxterm, I'd suggest adding
'GRUB_TERMINAL=console' to /etc/default/grub and using that to
generate grub.cfg. This fixes my qemu booting issue and is in line
with what the debian8 image did.

(Or maybe even do GRUB_TERMINAL=serial considering the kernel is told
to use the serial console.)