PROXY: Remove cache_timeout attribute from save_group()
    
    As this function already receives a struct sss_domain_info * parameter
    as argument, we can simply get the cache_timeout attribute by accessing
    domain->group_timeout.
    
    Related:
    https://fedorahosted.org/sssd/ticket/3134
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>