Optimize tasks for the container backend if defined

When the machinectl backend is in service there's no need to create a
tarball of the container rootfs. This change sets a fact for the backend
store and skips tasks that are unnecessary when using machinectl.

Change-Id: Ie759ae486157a31523b261041eb66bbb20f80e5a
Signed-off-by: Kevin Carter <kevin.carter@rackspace.com>

tasks/lxc_cache_create.yml

@@ -13,6 +13,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+- name: Set backingstore fact
+  set_fact:
+    container_backing_store: "{{ lxc_container_backing_store | default('None') }}"
+
 - name: Create LXC cache dir
   file:
     path: "{{ cache_path_fact }}"
@@ -23,17 +27,23 @@
   file:
     path: "{{ cache_path_fact }}/rootfs.tar.xz"
     state: "absent"
+  when:
+    - container_backing_store != 'machinectl'
 
 # This is using a shell command because the ansible archive module does not
 # provide for the options needed to properly create an LXC image archive.
 # Ansible will print a warning since this task calls 'tar' directly and we
-# suppress this warning with 'warn: no'.
+# suppress this warning with 'warn: no'. This task is not run when the backend
+# is set to "machinectl" because the "machinectl" backend directly clones the
+# base image instead of unpacking a container tarball.
 - name: Create lxc image
   shell: |
     tar -Opc -C {{ lxc_image_cache_path }} . | {{ lxc_xz_bin }} -T 0 -{{ lxc_image_compression_ratio }} -c - > rootfs.tar.xz
   args:
     chdir: "{{ cache_path_fact }}/"
     warn: no
+  when:
+    - container_backing_store != 'machinectl'
   tags:
     - skip_ansible_lint
 
@@ -79,10 +89,12 @@
   copy:
     content: "{{ cache_time }}"
     dest: "{{ cache_path_fact }}/build_id"
+  when:
+    - container_backing_store != 'machinectl'
 
 - include: "{{ item }}"
   static: no
   with_first_found:
     - files:
-        - "lxc_container_{{ lxc_container_backing_store | default('None') }}.yml"
+        - "lxc_container_{{ container_backing_store }}.yml"
       skip: true