workshop: Update docs and support default cloud image
    
    Update instructions on how to build images starting with Fedora 34 using
    kickstart files used by Fedora to build its cloud images.
    
    Change vagrant provisioning steps to support both prebuilt and default
    cloud images, removing the burden of maintaining boxes up-to-date, but
    also providing a way to build fresh images without external packer
    templates.
    
    Signed-off-by: Armando Neto <abiagion@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Francisco Trivino <ftrivino@redhat.com>
    Reviewed-By: Sumit Bose <sbose@redhat.com>