Fix nfs source crash when options change (#1264071)
    
    When stage2 is not nfs (eg. http) and the repo is on nfs the source
    setup code would get confused on the 2nd pass through, thinking it was a
    DRACUT_ISODIR that had been setup when in fact it was INSTALL_TREE. They
    both point to the same place.
    
    Resolves: rhbz#1264071