Make our vagrant-libvirt -based build system compatible with testing/sid's ruby-net-ssh

I’m told it currently requires the version from Jessie. If it ain’t broken, don’t burn it, right?

Feature Branch: feature/6354-vagrant-libvirt

Original created by @intrigeri on 11154 (Redmine)

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