(PUP-2916) yumrepo fails to honour custom reposdir

The yum.conf file wasn't being parsed for the reposdir which was causing the yumrepo provider to ignore the setting and just place the repo files into the default directory. Yum is then not seeing these repos as it has been configured to read files from the custom directory.

Also, self.repofiles was enumerating files from all directories that exist, i.e. the defaults directory and the custom directory. This created issues for puppet when the same repo existed in both directories. In reality, yum only parses the files in the custom directory if specified and ignores any files in the default directory.
A comment is needed here that explains the behavior - the code raises the question why "grabbing the resposdir.last" is the correct thing to do.