Bump coreos-assembler to f29
    
    Otherwise `rojig` gets created with `fc28` for `%{dist}`.
    
    Wanting to change the bare minimum to get this working, I created a new
    container with essentially:
    
        FROM quay.io/cgwalters/coreos-assembler:alpha
        RUN dnf distro-sync --releasever=29 -y
    
    And commits it as a squashed image. Really it's just using rpm-ostree in
    there, so we don't need all `coreos-assembler` to work.