Add squashfs_only parameter to lorax_options
    
    This will enable solution to the change proposal:
    https://fedoraproject.org/wiki/Changes/OptimizeSquashFSOnDVDByRemovingEXT4FilesystemImageLayer
    
    This change will reduce the image size and also
    is an additional step towards improving the reproducibility
    of the DVD image
    
    This change can be tested in Fedora RawHide:
    https://kojipkgs.fedoraproject.org/compose/rawhide/Fedora-Rawhide-20201030.n.0/compose/Server/x86_64/iso/
    
    Jira: RHELCMP-2900
    Signed-off-by: Bohdan Khomutskyi <bkhomuts@redhat.com>