https://bugs.launchpad.net/ubuntu/+source/cloud-init/+bug/1682160
http://pad.lv/1674766
    
=== Begin SRU Template ===
[Impact]
update-grub-legacy-ec2 fails on system with missing /etc/fstab.

[Test Case]

Mimic missing /etc/fstab config using lxc-proposed-snapshot from 
  https://git.launchpad.net/~smoser/cloud-init/+git/sru-info/tree/bin/lxc-proposed-snapshot

It publishes an image to lxd with proposed enabled and cloud-init upgraded.

$ for release in xenial yakkety zesty; do
$ ref=$release-proposed
$ lxc-proposed-snapshot --proposed --publish $release $ref
$ lxc init $ref $name
$ lxc start $name
$ sleep 10
$ lxc exec $name apt install grub-legacy-ec2
$ lxc exec $name mv /etc/fstab /etc/fstab.orig
$ lxc exec $name dpkg-reconfigure grub-legacy-ec2

# Ensure error message does not exist "This error is probably caused by an invalid /etc/fstab"

[Regression Potential] 
Low. oneliner in a script

[Other Info]
Upstream commit:
 https://git.launchpad.net/cloud-init/commit/?id=fd9f36267541

=== End SRU Template ===