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.