NSS: Use correct name for invalidating memory cache
    
    After refactoring of sysdb, we get and internal fully qualified
    name from backend in org.freedesktop.sssd.dataprovider_rev.initgrCheck
    Previously we got short name and we created fq name in
    nss_update_initgr_memcache. Memory cache still need to use short names
    if it was specified.
    
    This patch uses right name in different places.
    
    Reviewed-by: Petr Cech <pcech@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>