when a plugin depends on an attribute that doesn't have a provider, look for providers in parent attribute(s)
move lookup of parent providers for attributes from runner to provides_map
separate out different ways of gathering attribute providers into different methods
add methods for verifying and splitting attributes, fix behavior of find_closest_providers_for to look for a parent attribute that actually has providers, create new error Ohai::Exceptions::ProviderNotFound to indicate when an attribute exists in the mapping but doesn't have any listed providers
Merge pull request #240 from opscode/OC-9410

when a plugin depends on an attribute that doesn't have a provider, look for providers in parent attribute(s) and pick those to run instead.