Add libvirt as a vagrant provider

[plcp]

It's practical to use libvirt sometimes thanks to its support of kvm and ease of installation.

Thus this issue may be solved by providing an alternate Vagrantfile to test lightnion against debian/stretch64 (that supports libvirt as a provider).

Note that the vagrantbox we're using – ubuntu/bionic64 – is not available for libvirt.

[plcp]

Just started a stub here: https://github.com/spring-epfl/lighttor/compare/master...vagrant/stretch64

I suggest to have several configurations as shown here.

[wouterl]

I'm not sure if I really like that several configurations options tbh. I'd like to keep this as simple as possible. However, I am also completely not attached to the ubuntu VM. So if the debian/stretch64 one works well, I'd be happy if we change the Vagrant file to use stretch instead.

Then we can just use this approach that seems to be a bit easier to handle.

[plcp]

@wouterl we can effectively keep a single Vagrantfile if we only want to have different providers running the same vagrantbox, however we'll need at some point to bring some kind of configuration to provide a variety of setups and/or vagrantboxes for lightnion – useful for tests or picky users.

We can also keep a simple, single Vagrantfile in the root of the repository and later bring the bigger guns in a subdirectory where all the more advanced stuff happen – thus leaving a clear entry-point for someone that just want to git clone then vagrant up.

[plcp]

Note: debian/stretch64 won't be straightforward¹ as long as we don't support python 3.5 – see #5

¹we can install python 3.6+ on the VM from source or testing, but it's cumbersome.

[plcp]

Needs #5 to be fixed and is not a very high priority issue.