fix #3428: cloudinit: add parameter for upgrade on boot

fix #3428: cloudinit: add parameter for upgrade on boot

up until now, we did an automatic upgrade after the first boot in our
standard cloud-init config. This has been requested to be toggleable
several times [1][2]. With this patch, "package_upgrade" is disabled by
default, and needs to be enabled manually, diverging from the previous
behaviour.

[1] https://forum.proxmox.com/threads/how-to-prevent-automatic-apt-upgrade-during-the-first-boot-with-cloud-init.68472/
[2] https://forum.proxmox.com/threads/cloud-init-ohne-package-upgrade.123841/

Signed-off-by: Leo Nunner <l.nunner@proxmox.com>