Delete the Vagrant VM used for an ISO build once it is finished

Originally created by @intrigeri on #11981 (Redmine)

Given #11980 (closed), this is needed to avoid wasting lots of disk space.

Feature Branch: wip/11972-use-vagrant-in-jenkins

Edited May 15, 2020 by intrigeri

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information