Avoid keeping re-downloading container caches

When the cache tarball gets deleted at the end the task set in
playbooks/roles/lxc_hosts/tasks/lxc_cache.yml
then it will be downloaded again at subsequent runs, which makes the
download task not idempotent, and needlessly slow.

The unarchive task also needs to only be executed if the download task
performed a change.

Ensure that the location the download is placed and the directory it's extracted
to are different.

Change-Id: I4e433e82499f5cfa0b5dde44a6aa7bab113d9e0e

playbooks/roles/lxc_hosts/tasks/lxc_cache.yml

@@ -16,8 +16,9 @@
 - name: Download lxc cache(s)
   get_url:
     url: "{{ item.url }}"
-    dest: "/var/cache/lxc/{{ item.name }}"
+    dest: "/var/cache/lxc_{{ item.name }}"
     mode: "0644"
+    force: no
   register: cache_download
   until: cache_download|success
   with_items: lxc_container_caches
@@ -27,19 +28,12 @@
 
 - name: Move lxc cached image into place
   unarchive:
-    src: "/var/cache/lxc/{{ item.name }}"
+    src: "/var/cache/lxc_{{ item.name }}"
     dest: "/var/cache/lxc/"
     copy: "no"
   with_items: lxc_container_caches
+  when: cache_download|changed
   tags:
     - lxc-cache
     - lxc-cache-unarchive
 
-- name: Remove cache tarball
-  file:
-    path: "/var/cache/lxc/{{ item.name }}"
-    state: absent
-  with_items: lxc_container_caches
-  tags:
-    - lxc-cache
-    - lxc-cache-remove