dc-client/tune_sssd.sh: fix ad.conf creating

dc-client/tune_sssd.sh

@@ -90,14 +90,14 @@ debug_level = 1
 EOF
 
 if [ "$(epm print info -d)" = "ALTLinux" ] ; then
-cat <<EOF >/etc/sssd/conf.d/ad.conf || fatal
+cat <<EOF >>/etc/sssd/conf.d/ad.conf || fatal
 
 ; Should be used until ALT#34124 will be resolved
 user = root
 EOF
 fi
 
-cat <<EOF >/etc/sssd/conf.d/ad.conf || fatal
+cat <<EOF >>/etc/sssd/conf.d/ad.conf || fatal
 
 [nss]
 debug_level = 1