ttcn3-bsc: Update osmo-bsc cfg files to use mgw pooling VTY commands
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.
