net: add osmo-bsc-nat

Make STP_CN_IP and STP_RAN_IP configurable. If they are not set to the
same IP, run a second OsmoSTP and OsmoBSCNAT to connect both OsmoSTP.

Related: OS#2545
Change-Id: I8bfbd00bfeeebc6e1b87a8cd6333212db4380653
7 files changed