Build and use osmo-bsc_mgcp from openbsc.git instead of osmo-mgw.git

In osmo-mgw.git 31b4729f2731c747b8b33c4646dd5ade2ace29bc, osmo-bsc_mgcp
was dropped and the one from openbsc.git must be used.

Change-Id: Ib1c46baf4d36ab18f8064a2234a0ba28a1e7d2c2
diff --git a/src/osmo_gsm_tester/osmo_mgcpgw.py b/src/osmo_gsm_tester/osmo_mgcpgw.py
index 82b9149..668e4ce 100644
--- a/src/osmo_gsm_tester/osmo_mgcpgw.py
+++ b/src/osmo_gsm_tester/osmo_mgcpgw.py
@@ -38,7 +38,7 @@
         self.log('Starting osmo-mgcpgw')
         self.run_dir = util.Dir(self.suite_run.get_test_run_dir().new_dir(self.name()))
         self.configure()
-        inst = util.Dir(os.path.abspath(self.suite_run.trial.get_inst('osmo-mgw')))
+        inst = util.Dir(os.path.abspath(self.suite_run.trial.get_inst('osmo-nitb')))
         binary = inst.child('bin', 'osmo-bsc_mgcp')
         if not os.path.isfile(binary):
             raise RuntimeError('Binary missing: %r' % binary)