MDEV-31857 fix galera.galera_var_notify_ssl_ipv6

it was able to enable --ssl-verify-server-cert if explicily requested,
now it can also disable it, if explicitly requested.

mysql-test/std_data/wsrep_notify_ssl.sh

@@ -186,6 +186,8 @@ then
              "$ssl_verify_server_cert" = "on" ]
         then
             SSL_PARAM="$SSL_PARAM --ssl-verify-server-cert"
+        else
+            SSL_PARAM="$SSL_PARAM --disable-ssl-verify-server-cert"
         fi
     fi
 fi