ttcn3-sgsn: Use new 172.18.8.104 in emulated RNC
I forgot to push with last commit the change in IP address using the new
IP address in the emulated HNBGW. Doing it here to avoid test breakage.
Fixes: 594e021b9b699600a2f89a16676b05446a64fbdc
Change-Id: I20801e22bf20d2b2e67697fbf15811fe8eaa2996
