check valid repos in ks

check valid repos in ks

to avoid confused error message like:

    $ touch touch.ks
    $ sudo mic cr raw touch.ks
    Warning: No module named zypp, skip plugin backend/zypppkgmgr
    Info: Retrieving repo metadata:
     DONE

    Error <config>: Please specify a valid arch, your choise can be:

<Found by Marko>
thanks