Add ability to run v6 plugins

Modifies Ohai::System#all_plugins to work with new Ohai internals and adds deprecation message for this method. Also modifies Ohai::System#run_plugins to run v6 plugins that depend only on v6 plugins before running v7 plugins that depend only on v7 plugins. No testing/implementation for combinations of v6 and v7 plugins.

@danielsdeleo @lamont-granquist @sersut If you could take a look, that would be great!

(Note: locally pulled in OC-9012, and this branch does not have the most current changes there, yet.)