Use metadataCurrent() to check if need_download_mdFile

Use the yumrepo metadataCurrent() to check if needs to download mdFile.
Checking withinCacheAge(localMDFile,metadata_expire) not working in some systems (centos 5) where the repomd.xml keeps the original time attributes, must use the cachecookie file, but not directly, so I changed to use metadataCurrent().