net: add mgw for osmo-bsc-nat

Start the OsmoBSCNAT's own instance of OsmoMGW. Launch it on a separate
IP, such as the other MGWs, so it's easy to differentiate in wireshark
which traffic relates to which MGW.

Related: https://osmocom.org/projects/osmo-bscnat/wiki/Ladder_diagrams_for_key_procedures
Change-Id: Iaeebb693c81d3aa4b1a46d2543c9872bd49b87c3
3 files changed