Rewrite Vagrantfile to multi VM format

This makes it easy to start multiple VMs but also improves debugability
because the VM will be named foreman-bats_centos64 in libvirt as well.
Run tests via /vagrant
Install to /usr to avoid $PATH problems
Support a local foreman-installer

This allows testing pull requests against foreman-installer and its
submodules.
Add f20 to the boxes