Multiple disk image support

[iam-TJ]

This continues the original author's issue 91

sciurus#91

[iam-TJ]

Please test and report back. Patches are in the "multi-disk" branch for now. Because I need to get on with other work that this delayed I've only tested extensively with ubuntu/bionic64 virtualbox to libvirt, and ensured the test/test.rb passes.

On Ubuntu 18.04, to prevent over-writing the system-installed vagrant-mutate, I use a bind mount to replace it with the git repo for testing thus:

cd /home/all/SourceCode
git clone https://github.com/iam-TJ/vagrant-mutate.git
cd vagrant-mutate
git checkout multi-disk
sudo mount --bind $(pwd) /usr/share/rubygems-integration/all/gems/vagrant-mutate-1.2.0

If you want Ubuntu boxes using cloud-init to work with vagrant-libvirt you'll also need my minor patches to vagrant-libvirt (instructions on amending the Vagrantfile in the pull request vagrant-libvirt/vagrant-libvirt#907):

https://github.com/iam-TJ/vagrant-libvirt/tree/cdrom-set-type