The LXC container creation playbook is one of the longest-running playbooks
in the repository. It generally takes 15-17 minutes to run during the gate
jobs. Much of this time is spent updating each container with the latest
packages.
This patch causes the LXC cached image to be updated one time before that
image is used for all of the containers. It reduces the amount of times the
updates actually run and this shortens the time it takes to complete the
playbook.
The updates to the cached image will only occur if a new cache image has
just been downloaded.
Partial-bug: 1489169
Change-Id: Iba64f9a3aeb999907088f2a99e8904700074550b
05ae112e20