OsmoBSC has supported this VTY interface since more than a year ago.
Let's update the config files to use the new "mgw" node.
The recently submitted VTY commands without the redundant "mgw" prefix
are still not used here in order to have the config file work with
latest release which still doesn't support those.
Change-Id: Iabf117f9e6de02cac16e44d9a0ca32a30d71847c
Related: SYS#5987