Updated Vagrantfile's box_url, added private IP, added berkshelf plugin.

I had some trouble running `vagrant up`, so I made a few changes:

1. Added `config.berkshelf.enabled = true` such that all of the necessary cookbooks are automatically added.
2. Switching to a standard ubuntu cloud raring 64-bit image since I think this is more robust than a dropbox hosted one.
3. Moved chef's 'lxc' recipe inside of the lxc provider block. This was preventing me from using vagrant with virtualbox unless I added 'lxc' to Berksfile. Now, I didn't test if this still works with running vagrant inside of lxc. Can someone check?
4. Set a fixed private IP (this is not ideal, but is the simplest solution) for now until automatic private IP assignment and host file management bugs can be sorted out. This is the approach that dokku's current Vagrantfile uses (https://github.com/progrium/dokku/blob/master/Vagrantfile).