move all helper functions from providers into their own library

- we had 7 providers, each of which were duplicating code for find_id and find_value functions
- we had separate functions for find_id, find_creds_id etc

This pull basically:
- consolidates shared code into a separate library, helpers.rb
- converts all 'value' lookups to use the single function find_value, which takes an arg of the value name to return
- removes extra unneeded functions
- mixes the library into the Chef::Provider class
- retabs to 2 spaces to clean up files