Use Fog.providers and Fog.services to require files and instantiate services

Using the information in Fog.providers and the Fog.services to do the requires and instantiation in compute.rb, storage.rb, etc should help dry things a bit.  It might help with [1].  This will prevent the need to monkey patch compute and storage when creating a simple ad hoc provider which is my primary motivation. This passes all tests.  I have also made 2 minor corrections. 

[1] https://codeclimate.com/github/fog/fog/Fog::Compute#duplication