Merge remote-tracking branch 'upstream/master' into sync-upstream

Fix errors with unattended upgrades.

Recently, an update to the grub package caused the build to hang, as a
configuration file changed. This lets the newer package overwrite the
existing file.

Merge branches 'live-restore' and 'chrony'.

Merge pull request #9 from AdvMicrogrid/sync-with-upstream

Sync with upstream

Updated Changelog for v20190109

Update README and CHANGELOG for v25 release (awslabs#89)

Use apt-get instead of apt to upgrade.

Builds were stalling here, and this seems to resolve it.

Merge pull request #2 from AdvMicrogrid/ubuntu-base

Use Ubuntu 18.04 as source AMI

Scrub temporary build-time keypair

This keypair is an artifact of the build process- it's a temporary key and does not represent a secrity risk. However, it is a best practice to remove these keys before publishing, so we've added a step to remove it for all future AMI

Add ability to modify node root volume size

* Add ability modify root volume size