Made change to read metadata.json before metadata.rb 

In addition, fixed the argument to from_json to be a filename instead of the actual content.

The issue that led to this change is the desire to use metadata.json discussed in this PR:

https://github.com/opscode/test-kitchen/pull/212

In order for chef-zero to use the generated metadata.json we had to make the following change to prefer the .json version over .rb. We also discovered that the from_json method requires a file path instead of the JSON contents which were originally being passed to it.