Use lazy defaults for Chef Config

The default for some Chef Config objects (like file_cache_path) was
Dir.mktmpdir. Since this file is evaluated at runtime, the temporary
directory was being created, regardless of whether Ridley was invoked...
Simply requiring the file would cause a tmpdir to be created.
Merge pull request #276 from RiotGames/sethvargo/delayed_defaults

Use lazy defaults for Chef Config