Discussion on security updates

[mcapuccini]

I thought to open this issue to follow discussion on image security updated. Pointers:

Current status:

• We update images daily, however scripts won't import them
• We have unattended upgrades cron job on each node

Basic devel idea (to incorporate on kn apply):

• [x] Trigger unattended upgrades on boot time
• [ ] Check if reboot is needed (ssh ubuntu@master '[ -f /var/run/reboot-required ]')
• [ ] If reboot is needed inform user, and ask to run kn image-update

Drawback: user will need to destroy and recreate cluster when getting warning (and caring about security)

Advanced ideas:

• kn reboot to reboot the cluster
• Modify image CI so that a new image is created only when reboot is needed

@carmat88, @andersla please feel free to add on this.