tests: move common cloud-init config to a unified file, rename some base distribution.

tests/cloud-init/ubuntu24-desktop.user-data.yml

@@ -2,23 +2,6 @@
 
 # Based on https://github.com/canonical/autoinstall-desktop
 
-# https://github.com/canonical/ubuntu-desktop-provision/blob/main/README.md
-
-hostname: ${hostname}
-
-ssh_pwauth: true
-users:
-  - name: ${username}
-    plain_text_passwd: ${password}
-    shell: /bin/bash
-    ssh_authorized_keys:
-      - ${ssh_key}
-    lock_passwd: false
-    sudo: ALL=(ALL) NOPASSWD:ALL
-
-package_update: true
-package_upgrade: true
-package_reboot_if_required: false
 packages:
   - apparmor-profiles
   - build-essential