Fix bug with filename-only option

Hello,

Thanks for the recipe.  I ran into a an error when providing just a simple hash of 'certalias' -> 'filename' in my 'certificate_files' attribute. It's due to 'certificate_file' not being in scope at line 51. Looks like it's probably a copy/paste error. I fixed it and did a quick test in my environment. Thought you might want to know.

- Mike