Move more branch selection to helper functions
    
    Works with versions bound to a particular Fedora release are branched
    and built for each release.  The builders are fed a list of 'approved'
    branches, and monitor activity in each repo for each approved branch.
    
    Branches of guides *must* follow the project-wide convention of "fN" for
    their content to be picked up.  "F20" doesn't count, "f20-latest" is
    ignored, "20" is not published.  The builders care about "f20" or "f22".
    
    Previously, this branch list was more or less manually maintained. This
    commit changes that to query Fedora pkgdb instead, offloading the
    maintenance of the list to releng, because they're doing it anyway.
    
    There is one opportunity for docs-level policy enforcement: which
    branches are published and which are not.  Initially, this is configured
    to publish four versions of a given work, although this is of course up
    for discussion:
            - The release under development (N+1, ie Alpha, Beta docs)
            - The current primary release (N)
            - The current maintenance release (N-1)
            - The most recent EOL release (N-2, to help those upgrading)
    
    There's surely a more pythonic way to grab the latest EOL release than
    my overiterating, but I couldn't figure it out.  It works, meh.