cloudinit: restore previous default for package upgrades

cloudinit: restore previous default for package upgrades

Commit efa3355d ("fix #3428: cloudinit: add parameter for upgrade on
boot") changed the default, but this is a breaking change. The bug
report was only about making the option configurable.

The commit doesn't give an explicit reason for why, and arguably,
doing the upgrade is not an issue for most users. It also leads to a
different cloud-init instance ID, because of the different setting,
which in turn leads to ssh host key regeneration within the VM.

Reported-by: Friedrich Weber <f.weber@proxmox.com>
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>