Improve upgrades and reboots

- Tidy up tasks in package_upgrade.yml
- Add warning comment to reboot.yml playbook
- Take package upgrade tasks out of the common playbook so that
  they don't always run
- Add debian-goodies package for 'checkrestart' utility
Merge pull request #42 from dpla/improve-upgrade-and-reboot

Improve upgrades and reboots