gbproxy: Start to allocate NRIs to SGSNs

We change the osmo-gbproxy.cfg to use NRI 3 for SGSN-0 (NSEI 101) and
NRI 4 for SGSN-1 (NSEI 102).

We also add the capability to GBProxy_Tests to allocate P-TMSI with
a specific NRI and default to the first NRI of SGSN-0 so all tests
written still for a single-SGSN-non-pool scenario pass without any
modification.

Related: OS#4472, SYS#5002
Change-Id: I15f78dca5ac31a1c71f086d3e92a70e900eae250
diff --git a/gbproxy/osmo-gbproxy.cfg b/gbproxy/osmo-gbproxy.cfg
index 1c6caa9..5789e3a 100644
--- a/gbproxy/osmo-gbproxy.cfg
+++ b/gbproxy/osmo-gbproxy.cfg
@@ -6,10 +6,13 @@
  no login
 !
 gbproxy
+ nri bitlen 5
 sgsn nsei 101
  name first
+ nri add 3
 sgsn nsei 102
  name second
+ nri add 4
 ns
  bind udp local
   listen 127.0.0.1 23000