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.
Fixing an integration test that was assuming the chefignore would be only located in the cookbooks directory
Fixing indentation
Rename the method to be more descriptive
Use an attr_accessor instead of referencing the instance variable directly
Update the unit tests to use the files names instead of the number of files
CHEF-4203: Add documentation updates