Use LXC reverse proxy in OpenStack-CI

This patch implements the use of the images.linuxcontainers.org
reverse proxy when in nodepool. The same implementation as used
for MariaDB and UCA is used - this is not optimal as nodepool
settings may change, but this will do for now.

Depends-On: Ia6b149f157e5697d36fdea3dc65cb2ac4815a1a4
Change-Id: I558663e31264e35a4916c7b636a09cee3910408b

tests/roles/bootstrap-host/tasks/prepare_aio_config.yml

@@ -77,6 +77,13 @@
     - nodepool_dir.stat.exists
     - bootstrap_host_ubuntu_repo is defined
 
+- name: Set the LXC image repository URL in OpenStack-CI
+  set_fact:
+    lxc_image_cache_server: "{{ bootstrap_host_ubuntu_repo | netloc_no_port }}:8080/images.linuxcontainers"
+  when:
+    - nodepool_dir.stat.exists | bool
+    - bootstrap_host_ubuntu_repo is defined
+
 - name: Set the package cache timeout to 60 mins in OpenStack-CI
   set_fact:
     cache_timeout: 3600

tests/roles/bootstrap-host/templates/user_variables.aio.yml.j2

@@ -161,6 +161,11 @@ galera_repo_url: {{ galera_repo_url }}
 galera_client_apt_repo_url: {{ galera_repo_url }}
 {% endif %}
 
+{% if lxc_image_cache_server is defined %}
+## images.linuxcontainers.org reverse proxy
+lxc_image_cache_server: {{ lxc_image_cache_server }}
+{% endif %}
+
 {% if cache_timeout is defined %}
 ## Package cache timeout
 cache_timeout: {{ cache_timeout }}