chore(ci): add apt mirror support for ubuntu in cloud-init

[universal-itengineer]

Description

This PR adds APT mirror support for Ubuntu in e2e tests and cloud-init configurations.

Changes

• CI (e2e-reusable-pipeline.yml): Added apt_mirror_enabled, apt_mirror_name, and apt_mirror_url input parameters with Hetzner mirror as default
• CI (e2e-matrix.yml): Enabled aptMirror in Deckhouse config for replicated and NFS e2e test jobs
• Cloud-init (_cloudinit.tpl): New template for configuring APT repositories in VM cloud-init
• NFS VM: Updated to use the new cloud-init module with mirror support

Why do we need it, and what problem does it solve?

E2e tests running on Hetzner infrastructure experience slow or failing package downloads from default Ubuntu repositories. Using a local Hetzner mirror (https://mirror.hetzner.com/ubuntu/packages) improves reliability and speed.

What is the expected result?

VMs created during e2e tests use Hetzner mirror for apt package downloads, resulting in faster and more reliable test execution on Hetzner infrastructure.

Checklist

• The code is covered by unit tests.
• e2e tests passed.
• Documentation updated according to the changes.
• Changes were tested in the Kubernetes cluster manually.

Changelog entries

section: ci
type: chore
summary: "Added APT mirror support for Ubuntu in e2e tests with Hetzner mirror as default."