bug: Debian preseed doesn't account for different regions

[hazzuk]

Prerequisites

• I have searched existing and closed issues

What version are you using?

n/a

Expected behavior

The Debian preseed should set the system's locale and timezone according to the user's preference.

Actual behavior

Currently these are hard-coded as GB values.

Describe the issue

d-i debian-installer/locale string en_GB.UTF-8
d-i keyboard-configuration/variant select British English
d-i keyboard-configuration/xkb-keymap select gb

d-i time/zone string Europe/London

Any additional notes

We can either change these value dynamically (like we do with the public SSH key), or we simply remove these lines and let the user manually select them during the Debian installer process.