Introduce osmux support for msc,bsc,mgw

Change-Id: Ief3518e77650f2cddb48357b07fe95346e46ee1b
diff --git a/src/osmo_gsm_tester/templates/osmo-mgw.cfg.tmpl b/src/osmo_gsm_tester/templates/osmo-mgw.cfg.tmpl
index 9e338e1..f91ab30 100644
--- a/src/osmo_gsm_tester/templates/osmo-mgw.cfg.tmpl
+++ b/src/osmo_gsm_tester/templates/osmo-mgw.cfg.tmpl
@@ -14,3 +14,4 @@
  number endpoints 31
  rtcp-omit
  rtp-accept-all 1
+ osmux ${mgw.use_osmux}