Updated vagrant environment

cc: @wizonesolutions

Since we've seen a couple of people (e.g. #3150) who had trouble getting the vagrant environment set up, I've updated the box so that all the provisioning happens at load up. I also added the use of vagrant cloud so we're no longer relying on the questionable bandwidth on my Dropbox. Added autocorrect of port forwarding if someone's already using port 3000. Expanded documentation for all the new features and options, and made a note in the instructions for config.json to be created before spinning up the vagrant box.