gbproxy: Configure a Second SGSN

This adds the required code changes and minimal configuration to both
osmo-gbproxy.cfg and GBProxy_Tests to have osmo-gbproxy connect to two
SGSNs.

No NRI mappings are configured and hence the second SGSN isn't really
used for anything except for bringing the NS-VCs and BVCs up.

Related: OS#4472, SYS#5002
Change-Id: Ib70f7c1a29f089957f882df0e9b05ee526224611
4 files changed