lxc_cache_preparation: Convert the LXC cache preparation to async task

openSUSE sometimes hangs when running this task. In order to debug this
failure we need to fire up this task and check for it's status on a
separate one. This way, we can avoid Jenkins timeouts and collect logs
properly when that tasks fails to complete within a reasonable amount of
time.

Change-Id: I3acc4436c8b70af097de188b9de2bb9f4e093a09

tasks/lxc_cache_preparation.yml

@@ -95,6 +95,19 @@
 - name: Prepare cached image setup commands
   shell: "chroot {{ lxc_image_cache_path }} /usr/local/bin/cache-prep-commands.sh > /var/log/lxc-cache-prep-commands.log 2>&1"
   changed_when: false
+  # It should only take a couple of minutes to prepare the cache. But if network is slow it might take longer.
+  # Lets be optimistic and set the limit to 10 minutes
+  async: 600
+  poll: 0
+  register: _lxc_cache_prepare_commands
+
+- name: Ensure that the LXC cache has been prepared
+  async_status:
+    jid: "{{ _lxc_cache_prepare_commands.ansible_job_id }}"
+  register: _lxc_cache_prepare_commands_result
+  until: _lxc_cache_prepare_commands_result.finished
+  delay: 10
+  retries: 60
 
 - name: Adjust sshd configuration in container
   lineinfile: