NSS: Don't set SocketUser/SocketGroup as "sssd" in sssd-nss.socket
    
    NSS service is always run as root, so there's no need to change its
    socket ownership to the sssd user.
    
    More than that, by setting up the SocketUser and SocketGroup to "sssd"
    a loop would be caused as the "sssd" would trigger an initgroups call
    during the NSS socket setup.
    
    The problem was found when starting up a machine with SSSD built with
    "--with-sssd-user=sssd" and having "sss" before "files" in the
    name-service switch.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3322
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>