Test our vagrant-libvirt -based build system with Jessie's Vagrant

Vagrant 1.6.5+dfsg1-1 got uploaded to Debian, and will likely be part of Jessie. As long as our main “easy” build system is using Vagrant, we should try to support that version.

Feature Branch: feature/6354-vagrant-libvirt

Parent Task: #6354 (closed)

Original created by @intrigeri on 8086 (Redmine)