Version entry URL for repository

Different set of packages may be necessary for various versions of the
RPM-based distros. Thus we need a way to know which one is indended to
be used (through the selection of the right url).

Also add a version number to the stem of the URL, so that later aliases
can be introduced to let one stay at previous versions.

Note that this depends on OPM/opm-web#5 to be fully functional.