t/autopkgtest-buildvm-ubuntu-cloud: add image cache

Ural Tunaboyurequested to merge
(removed):buildvm-ubuntu-cloud-image-cache into master

Adds a cache and associated option for downloaded images. Also improve output directory handling.

The three added options are:

  • --force-use-cached: Do not download the SHA256 sum of latest images to verify cached image.

LP: #2073938

Edited by Ural Tunaboyu

Merge request reports