Stop hacking /etc/default/grub to stay out of dpkg/apt's way

Changes that would have been edited into /etc/default/grub are now
fixed up by /etc/grub.d/01_ceph_qa_fixup.  This is not perfect; better
would be /etc/default/grub.d, but that's new enough that I don't
want to yet rely on it.

Continues fix to 4496

Fixes: #4496
Signed-off-by: Dan Mick <dan.mick@inktank.com>