mgw: Set osmux bind-ip(-v6) VTY configs

Until now we forgot to properly configure the osmux socket local ip, but
that was fine because 0.0.0.0 was taken as a default.
With addition of IPv6, this changed, and the socket is not bound unless
an IP address is set (to allow conditional use of IPv4, IPv6 or both).

Depends: osmo-mgw.git Change-Id I446cd7da217e9f4a74995d7784ae55dcc60a29b7
Change-Id: I4e0ea11ae7e55da600aec56bebf694f3ba0f7b72

mgw/osmo-mgw.cfg

@@ -47,3 +47,5 @@ mgcp
   number endpoints 31
   allow-transcoding
   osmux off
+  osmux bind-ip 127.0.0.1
+  osmux bind-ip-v6 ::1