Unattended grub configuration after kernel upgrade

I was able to get around this by using export DEBIAN_FRONTEND=noninteractive, before running apt-get upgrade -y.

On Ubuntu 16.04, this sequence should avoid the interactive dialog:

export DEBIAN_FRONTEND=noninteractive
apt-get update
apt-get upgrade -yq

Note that the -q flag is important.

(Reference: This question on devops)