tasks: lxc_cache_preparation: Log cache preparation command output

We already enable debug output (-x) in the shell script but we never
actually see this output during or after the cache preparation step
takes place. It's useful to log what exactly happened so we can look
for problems or find potential optimization opportunities so just log
the output to /var/log which is also being collected by the various CI
scripts.

Change-Id: I3d0ab31e052889b9ac03de9a3ccba680e840fca5

tasks/lxc_cache_preparation.yml

@@ -93,7 +93,7 @@
 # This task runs several commands against the cached image to speed up the
 # lxc_container_create playbook.
 - name: Prepare cached image setup commands
-  command: "chroot {{ lxc_image_cache_path }} /usr/local/bin/cache-prep-commands.sh"
+  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
 
 - name: Adjust sshd configuration in container