More reliable vagrant handling

This addresses some issues when running a local Vagrant cluster.
- Ensure the virtualbox provider is used (in util.sh), as the fedora box is virtualbox based.
- Use YUM install instead of salt bootstrap. Salt is in EPEL (http://docs.saltstack.com/en/latest/topics/installation/fedora.html), bootstrap does not support Fedora 20 and fails handling systemctl/systemd units (see supported operating systems https://github.com/saltstack/salt-bootstrap)