Add option to only run plugins providing requested attributes

* `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.