Add option to only run plugins providing requested attributes

##Summary

* `Ohai::System#all_plugins` takes an option whitelist filter
* `Ohai::ProvidesMap#find_providers_for` now finds all plugins at the
  specified nesting or deeper. For example, if a plugin provides
  "virtualization/networks" and no plugin provides "virtualization",
  then find_providers_for(["virtualization"]) would return `[nil]`. It
  now finds all providers deeper than the specified level in the map.

##Remaining

Since several of us are making changes in similar places simultaneously, I want to share this now. To complete the PBI, I think ohai's command line attribute selector (e.g., `ohai network`) needs to be integrated with this, which will help 3rd party plugin development by improving feedback time.