osmo-msc: Use mgw instead of mgcpgw in cfg

As of osmo-mgw 87203f2a3746d708f85ee75d978e2821bda58e6d, mgcpw commands
are deprectated.

Change-Id: Ib0220c9f7f6262234712c3195dda0e076c7dd390
diff --git a/src/osmo_gsm_tester/osmo_mgcpgw.py b/src/osmo_gsm_tester/osmo_mgcpgw.py
index 93c3a7c..665c151 100644
--- a/src/osmo_gsm_tester/osmo_mgcpgw.py
+++ b/src/osmo_gsm_tester/osmo_mgcpgw.py
@@ -81,7 +81,7 @@
         return self.ip_address.get('addr')
 
     def conf_for_msc(self):
-        return dict(mgcpgw=dict(ip_address=self.ip_address))
+        return dict(mgw=dict(ip_address=self.ip_address))
 
     def running(self):
         return not self.process.terminated()