Allow variable control over lxc_template task enviroment

This allows a deployer to configure a proxy server that is
only applied to these LXC cache tasks. This is sometimes preferable
to configuring such a proxy in the host's enviroment variables. It
may also be preferable to running a private LXC image server and specifying
it using the `lxc_image_cache_server` variable.

Change-Id: I884478c8bf699af2facbc106f745470eaf697ffd

defaults/main.yml

@@ -17,6 +17,11 @@
 # cache will be updated. By default this timeout is set to 1 day.
 lxc_cache_timeout: 86400
 
+# Environment to use when building LXC caches
+# You can set http_proxy environment variables here for example
+# if you would like to proxy all image downloads
+lxc_cache_environment: {}
+
 # Set the package install state for distribution and pip packages
 # Options are 'present' and 'latest'
 lxc_hosts_package_state: "latest"

releasenotes/notes/lxc_cache_environment-f14701a7f8f4b8ca.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - The variable ``lxc_cache_environment`` has been added. This dictionary
+    can be overridden by deployers to set HTTP proxy environment variables that
+    will be applied to all lxc container download tasks.

tasks/lxc_cache.yml

@@ -25,6 +25,7 @@
   delay: 10
   until: cache_download_primary | success
   failed_when: false
+  environment: "{{ lxc_cache_environment }}"
   tags:
     - lxc-cache
     - lxc-cache-download
@@ -41,6 +42,7 @@
   retries: 3
   delay: 10
   until: cache_download_secondary | success
+  environment: "{{ lxc_cache_environment }}"
   tags:
     - lxc-cache
     - lxc-cache-download

tasks/lxc_cache_create.yml

@@ -45,6 +45,7 @@
   retries: 3
   delay: 10
   until: cache_download|success
+  environment: "{{ lxc_cache_environment }}"
   when:
     - lxc_container_backing_store is defined
     - lxc_container_backing_store == 'overlayfs'
@@ -60,6 +61,7 @@
   retries: 3
   delay: 10
   until: cache_download|success
+  environment: "{{ lxc_cache_environment }}"
   when:
     - lxc_container_backing_store is defined
     - lxc_container_backing_store == 'lvm'