Fix bug with filename-only option

Hello,

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 and did a quick test in my environment.   Thought you might want to know.