Compress base container using parallel xz

The package for pxz is installed but was not being used, so
the base container compression was only using 1 core during
compression. This change allows pxz to be used which will
compress much faster when multiple cores are available.

Change-Id: I7eee214a69762b36c9bb2b23edad5688189bb967

tasks/lxc_cache_create.yml

@@ -25,7 +25,7 @@
 #  provide for the options needed to properly create an LXC image archive.
 - name: Create lxc image
   shell: |
-    tar -Opc -C /var/lib/lxc/LXC_NAME/rootfs . | xz -{{ lxc_image_compression_ratio }} -c - > rootfs.tar.xz
+    tar -Opc -C /var/lib/lxc/LXC_NAME/rootfs . | {{ lxc_xz_bin }} -{{ lxc_image_compression_ratio }} -c - > rootfs.tar.xz
   args:
     chdir: "{{ lxc_container_cache_path }}/{{ lxc_cache_map.distro }}/{{ lxc_cache_map.release }}/{{ lxc_cache_map.arch }}/default/"
   notify: Destroy base container

vars/redhat-7.yml

@@ -39,6 +39,8 @@ lxc_hosts_distro_packages:
   - redhat-lsb
   - xz
 
+lxc_xz_bin: xz
+
 lxc_cache_map:
   distro: centos
   arch: amd64

vars/ubuntu-14.04.yml

@@ -36,6 +36,8 @@ lxc_hosts_distro_packages:
   - python3-lxc
   - pxz
 
+lxc_xz_bin: pxz
+
 system_config_dir: "/etc/default"
 lxc_cache_map:
   distro: ubuntu

vars/ubuntu-16.04.yml

@@ -37,6 +37,8 @@ lxc_hosts_distro_packages:
   - python3-lxc
   - pxz
 
+lxc_xz_bin: pxz
+
 system_config_dir: "/etc/default"
 lxc_cache_map:
   distro: ubuntu