perftest: Disable unattended apt upgrades on outer host

[egmontkob-work]

In order to have accurate numbers from the perftests, we should disable unattended apt upgrades on the outer host. (We disable them in the inner LXC hosts.) This is a step to be written in README_SETUP.txt.

On the other hand, we don't want a publicly accessible machine to be running without receiving regular upgrades.

So we should figure out how to mutually lock between perftests and unattended apt upgrades.