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.fr.cfg b/gbproxy/osmo-gbproxy.fr.cfg
index e350526..11ab84b 100644
--- a/gbproxy/osmo-gbproxy.fr.cfg
+++ b/gbproxy/osmo-gbproxy.fr.cfg
@@ -49,7 +49,10 @@
   nsvc fr hdlcnet7 dlci 22 nsvci 7
   nsvc fr hdlcnet8 dlci 23 nsvci 8
 gbproxy
+ nri bitlen 5
 sgsn nsei 101
  name first
+ nri add 3
 sgsn nsei 102
  name second
+ nri add 4