Add OsmoSgsn class

Change-Id: Iec370a444998c632b3615ad1a6d6f67e64e2bfeb
diff --git a/src/osmo_gsm_tester/suite.py b/src/osmo_gsm_tester/suite.py
index 8c1d38d..0d5f97a 100644
--- a/src/osmo_gsm_tester/suite.py
+++ b/src/osmo_gsm_tester/suite.py
@@ -22,7 +22,7 @@
 import time
 import pprint
 from . import config, log, template, util, resource, schema, event_loop, test
-from . import osmo_nitb, osmo_hlr, osmo_mgcpgw, osmo_mgw, osmo_msc, osmo_bsc, osmo_stp, osmo_ggsn, modem, esme
+from . import osmo_nitb, osmo_hlr, osmo_mgcpgw, osmo_mgw, osmo_msc, osmo_bsc, osmo_stp, osmo_ggsn, osmo_sgsn, modem, esme
 
 class Timeout(Exception):
     pass
@@ -235,6 +235,11 @@
             ip_address = self.ip_address()
         return osmo_ggsn.OsmoGgsn(self, ip_address)
 
+    def sgsn(self, hlr, ggsn, ip_address=None):
+        if ip_address is None:
+            ip_address = self.ip_address()
+        return osmo_sgsn.OsmoSgsn(self, hlr, ggsn, ip_address)
+
     def mgcpgw(self, ip_address=None, bts_ip=None):
         if ip_address is None:
             ip_address = self.ip_address()