boostrap-os: use import_tasks instead of symlinks (#11508)

Working symlinks are dependant on git configuration (when using the playbook as
a git repository, which is common), precisely `git config
core.symlinks`.

While this is enabled by default, some company policies will disable it.

Instead, use import_tasks which should avoid that class of bugs.

roles/bootstrap-os/tasks/ubuntu.yml

@@ -1 +0,0 @@
-debian.yml

roles/bootstrap-os/tasks/ubuntu.yml

@@ -0,0 +1,21 @@
+---
+- name: Import Debian bootstrap
+  import_tasks: debian.yml
+
+- name: Check unattended-upgrades file exist
+  stat:
+    path: /etc/apt/apt.conf.d/50unattended-upgrades
+  register: unattended_upgrades_file_stat
+  when:
+    - ubuntu_kernel_unattended_upgrades_disabled
+
+- name: Disable kernel unattended-upgrades
+  lineinfile:
+    path: "{{ unattended_upgrades_file_stat.stat.path }}"
+    insertafter: "Unattended-Upgrade::Package-Blacklist"
+    line: '"linux-";'
+    state: present
+  become: true
+  when:
+    - ubuntu_kernel_unattended_upgrades_disabled
+    - unattended_upgrades_file_stat.stat.exists