Added support for vagrant-libvirt

[MarkusOstermayer]

Hey, this PR adds support for libvirt via the vagrant-libvirt provider for vagrant.
This PR closes #237

For implementing the provider, I followed the provided documentation and copied what was done for the virtualbox-provider.
I added support for this provider for the GOAD, the GOAD-Mini and the GOAD-Light lab.
Since the boxes StefanScherer/windows_2016/StefanScherer/windows_2019 were not available for libvirt, I used jborean93/WindowsServer2016/jborean93/WindowsServer2019.

I also added documentation for the new libvirt provider.
If I missed something, please let me know and I'll add/fix it.

[t3rp]

This is great, thank you for doing this! I deployed GOAD (full) using your PR without issue on libvirt 10.9.0. Two notes:

1. There is a non-blocking error [fog][WARNING] Unrecognized arguments: libvirt_ip_command from Vagrant
2. libvirt should be added to the ALLOWED_PROVIDERS in goad/utils.py (I don't have permission to modify your PR)

# providers
LIBVIRT = 'libvirt'
AWS = 'aws'
VIRTUALBOX = 'virtualbox'
AZURE = 'azure'
VMWARE = 'vmware'
VMWARE_ESXI = 'vmware_esxi'
PROXMOX = 'proxmox'
LUDUS = 'ludus'
ALLOWED_PROVIDERS = [AWS, VIRTUALBOX, AZURE, VMWARE, VMWARE_ESXI, PROXMOX, LUDUS]

[MarkusOstermayer]

Thanks for testing and checking the code changes.
I added it to the ALLOWED_PROVIDERS.

The error that you mentioned seems to be an issue within vagrant-libvirt (see vagrant-libvirt/vagrant-libvirt#1831 (comment)).
I am currently not able to reproduce it, but I also noticed that I am on a rather old version of vagrant (2.3.8).
I'll try to update vagrant in the following days and look into it 😄