doc: do not use random ip-address for MGW

The example configs suggest to use a random ip-address as MGW address.
Lets use a loopback address here. This will suit the usual case where
MGW and MSC run together on the same machine.

Change-Id: Ie2b2094fdcfed45353d9ba22cb07eed626fd143c
diff --git a/tests/test_nodes.vty b/tests/test_nodes.vty
index 5a81c61..edb891b 100644
--- a/tests/test_nodes.vty
+++ b/tests/test_nodes.vty
@@ -160,7 +160,7 @@
  auth-tuple-max-reuse-count 3
  auth-tuple-reuse-on-error 1
  mgw local-port 2728
- mgw remote-ip 10.23.24.1
+ mgw remote-ip 127.0.0.1
  mgw remote-port 2427
 mncc-int
  default-codec tch-f fr