CHEF-4203: Honor the chefignore file for knife cookbook test

In the case that a standalone cookbook exists that is using bundler, a large amount of gems can be added to the current directory. This commit reads the chefignore file and avoid having to scan through the ruby files that are in the bundler created gems directory.

https://tickets.opscode.com/browse/CHEF-4203

This builds upon https://github.com/opscode/chef/pull/777 and allows for the chefignore file to be honored if the cookbook exists outside of the chef_repo directory structure.