Implement base container for overlayfs-backed containers

Since overlayfs is now part of the mainstream kernel, it makes sense
to take advantage of it to speed up the container creation process.

This patch implements a base container to be used in the container
creation process if 'overlayfs' is set as the container backing
store.

Change-Id: I20123b83af10c0890f4d5945b9fb230acd664213

defaults/main.yml

@@ -22,6 +22,11 @@ lxc_architecture_mapping:
 lxc_container_directory: "/var/lib/lxc"
 lxc_container_cache_path: "/var/cache/lxc/download"
 
+# The container backing store can be set to 'overlayfs' to use overlayfs
+# This should only be done for production use with a linux kernel > 3.14
+# which is when overlayfs was merged into the mainline kernel
+# lxc_container_backing_store: overlayfs
+
 # lxc container net network
 lxc_net_bridge: lxcbr0
 lxc_net_bridge_port: none

releasenotes/notes/base-container-overlayfs-ec7eeda2f5807e96.yaml

@@ -0,0 +1,11 @@
+---
+features:
+  - The container cache preparation process now allows ``overlayfs`` to be set
+    as the ``lxc_container_backing_store``. When this is set a base container
+    will be created using a name of the form
+    `<linux-distribution>`-`distribution-release>`-`<host-cpu-architecture>`.
+    The container will be stopped as it is not used for anything except to be
+    a backing store for all other containers which will be based on a snapshot
+    of the base container. The ``overlayfs`` backing store is not recommended
+    to be used for production unless the host kernel version is 3.18 or
+    higher.

tasks/lxc_cache_create.yml

@@ -33,3 +33,18 @@
     - skip_ansible_lint
     - lxc-cache
     - lxc-image-cache-create
+
+- name: Create base container to use for overlayfs containers
+  lxc_container:
+    name: "{{ lxc_cache_map.distro }}-{{ lxc_cache_map.release }}-{{ lxc_cache_map.arch }}"
+    template: "download"
+    state: stopped
+    backing_store: "dir"
+    template_options: "{{ lxc_cache_download_template_options }}"
+  register: cache_download
+  retries: 3
+  delay: 10
+  until: cache_download|success
+  when:
+    - lxc_container_backing_store is defined
+    - lxc_container_backing_store == 'overlayfs'