templates: osmo-bsc.cfg.tmpl: migrate to separate 'mgw' node

This commit fixes the following deprecation warnings:

% MGCP commands outside of 'mgw' nodes are deprecated.
  You should consider reading the User Manual and migrating to 'mgw' node.

Change-Id: Ic2b3c8a6775b6a34bfc03e0643a34772ae789139
diff --git a/src/osmo_gsm_tester/templates/osmo-bsc.cfg.tmpl b/src/osmo_gsm_tester/templates/osmo-bsc.cfg.tmpl
index b40b6f4..64adffe 100644
--- a/src/osmo_gsm_tester/templates/osmo-bsc.cfg.tmpl
+++ b/src/osmo_gsm_tester/templates/osmo-bsc.cfg.tmpl
@@ -112,13 +112,14 @@
 %  endfor
 % endfor
 %endfor
+ mgw 0
+  local-ip ${bsc.ip_address.addr}
+  remote-ip ${mgw.ip_address.addr}
+  remote-port 2427
 msc
  core-mobile-country-code ${bsc.net.mcc}
  core-mobile-network-code ${bsc.net.mnc}
  allow-emergency allow
- mgw remote-ip ${mgw.ip_address.addr}
- mgw remote-port 2427
- mgw local-ip ${bsc.ip_address.addr}
  codec-list ${' '.join(bsc.net.codec_list)}
  amr-config 12_2k forbidden
  amr-config 10_2k forbidden