Make the cache prep timeout configurable

In some environments it may be desirable to adjust the
cache timeout. This makes that possible.

Change-Id: I733887f16d844f07457c80c3c6936f668e12ce86

defaults/main.yml

@@ -131,6 +131,12 @@ lxc_cache_prep_post_commands: '## post command skipped ##'
 # List of packages to be installed into the base container cache
 lxc_cache_distro_packages: "{{ _lxc_cache_distro_packages }}"
 
+# The maximum amount of time (in seconds) to wait until failing the cache
+# preparation process. This is necessary to mitigate the issue that can
+# arise where the cache prep hangs and never fails.
+# The value is specified in seconds, with the default being 10 minutes.
+lxc_cache_prep_timeout: 600
+
 # Set the servers to download LXC images from
 # NOTE(mhayden): The main images.linuxcontainers.org site will redirect
 # requests to (us|uk).images.linuxcontainers.org upon the first request. We

releasenotes/notes/lxc-cache-prep-timeout-97dc18882f7b1e76.yaml

@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    The maximum amount of time to wait until forcibly failing the
+    LXC cache preparation process is now configurable using the
+    ``lxc_cache_prep_timeout`` variable. The value is specified
+    in seconds, with the default being 10 minutes.

tasks/lxc_cache_preparation.yml

@@ -110,7 +110,7 @@
   changed_when: false
   # It should only take a couple of minutes to prepare the cache. But if network is slow it might take longer.
   # Lets be optimistic and set the limit to 10 minutes
-  async: 600
+  async: "{{ lxc_cache_prep_timeout }}"
   poll: 0
   register: _lxc_cache_prepare_commands
 
@@ -133,7 +133,7 @@
   register: _lxc_cache_prepare_commands_result
   until: _lxc_cache_prepare_commands_result.finished
   delay: 10
-  retries: 60
+  retries: "{{ lxc_cache_prep_timeout // 10 }}"
 
 - name: Remove requiretty for sudo on centos
   template: