[createiso] Move runroot work to separate script
    
    Instead of running a long command line in the runroot (or locally), move
    all that work into a separate script that will be installed. This means
    chroot will need to install pungi.
    
    Everything should work as it did before. The only exception to this is
    that there is logic to find lorax templates instead of harcoding the
    location. This is done using a separate script.
    
    Related: #230
    Fixes: #231
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>