Allow setting runroot_method per Pungi phase.
    
    The `runroot_method` now accepts `dict` value with phase name as a key
    and runroot method as a value. For backward compatibility, the `str`
    value is still supported.
    
    The new `global_runroot_method` option has been added which defines
    the runroot method in case it is not set in `dict` in the `runroot_method`.
    
    This commit allows running `createiso` phase locally while keeping the other
    phases in Koji.
    
    Signed-off-by: Jan Kaluza <jkaluza@redhat.com>