tweak desktop creation commands and comments for F21 and F22
    
    Summary:
    For F22 we don't need to remove firewalld, as the approach to
    flavor configurations was changed and the conflict wasn't
    there any more. On the other hand for F21 we still do, and
    we need to use yum, not dnf. So make the command run inside
    virt-builder conditional on $VERSION. I figure we still might
    want to create F21 images if we want to test F21->F23 upgrades.
    
    Test Plan:
    I built the new set of HDDs on BOS with this change,
    so we can see if the upgrade tests all run as expected. To
    check the F21 images I guess we'd have to poke at 'em with
    guestfish or something.
    
    Reviewers: jskladan, garretraziel
    
    Reviewed By: garretraziel
    
    Subscribers: tflink
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D572