OHAI-315: Added virtualbox plugin + cloud properties

* virtualbox uses the VBoxControl program to enumerate guest VM properties. Note that it must be run as root.
* it also uses either the sniffed information, or the default ohai[:network] tree, to guess the public/private nature of its interfaces. It works as follows:
  - anything in 33.33.xx.xx is by convention a host_only private_ip
  - anything in 10.xx.xx.xx, 172.{16-32}.xx.xx, or 192.168.xx.xx (the ipv4 private address spaces) are private_ip (non-host_only)
  - everything else is public
  with this exception:
  - if no public_ip's are defined, the first non-host_only ip is relocated to public_ips, under the assumption that is the interface open to the host machine and in such a universe should be treated as private
  I'm not in love with this scheme, but we want something that works for the reasonable case and lets you mimic a typical cloud machine.
* The ohai[:cloud] parameters are set directly from the virtualbox heuristics