Make use of single configuration point for SELinux
    
    For now, FreeIPA supports SELinux things as they are in RedHat/Fedora.
    But different distributions may have their own SELinux customizations.
    
    This moves SELinux configuration out to platform constants:
    - SELINUX_MCS_MAX
    - SELINUX_MCS_REGEX
    - SELINUX_MLS_MAX
    - SELINUX_MLS_REGEX
    - SELINUX_USER_REGEX
    - SELINUX_USERMAP_DEFAULT
    - SELINUX_USERMAP_ORDER
    
    and applies corresponding changes to the test code.
    
    Fixes: https://pagure.io/freeipa/issue/7996
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>