Update locate_config_file scope to reflect its change to a Class method

The locate_config_file was changed to a class method on 2013-10-09:
https://github.com/opscode/chef/commit/2a214bcff7174b61129a7a8a642b8df5b5f4afee#diff-1220e994288a7f7d9ae83196ce7fad9eL317

* Fixes scope, and add abbreviated assignment for config[:config_file]
* Adds tests for two GreenAdnSecure Module methods

Refactor TestKnifeBlockList

* Removes ! in front of File.symlink?
  ** Test now fails only if knife.rb is not a symlink
* Set @chef_path to fallback to ~/.chef if WORKSPACE not defined
  ** This could also be sourced from locate_config_file which
     checks in more locations ($KNIFE_HOME, pwd, $HOME)