Refactor config file:
1. Change the layout of the config file to separate provisoner options
(vagrant_options) and layout
2. Rename config.json to config.json.example, allow users to safely
customize
Rakefile improvements to be more user friendly:  Copy config example to config.json if none exists, support more vagrant commands, and handle new config.json namespace
Require chef-metal 0.4.0 or greater to resolve the CentOS PV detection issue
Move the installer package filename out to an environment variable
Documentation to match reality
Merge pull request #8 from irvingpop/irving/config_refactor

config.json refactor