manifest: Use hardcoded passwd/group data
    
    Currently Fedora rel-eng does separate repositories for the different
    releases.  This means that rpm-ostree can't propagate the cached
    /usr/lib/passwd and /usr/lib/group from previous trees.
    
    Regardless, we'll have continual problems where people want to make
    their own trees and rebase to them.  In order to support this, there's
    no other current solution than hardcoding them here.