MSC_ConnectionHandler: allow to use IPV4 as default

When the BSC sends a CRCX without an IP address in it, the testcase will
automatically assign an IPV6 address in the response. However, this
breaks compatibility with older versions of osmo-bsc that do not have
IPV6 support. Lets add a module parameter in order to be able to use
IPV4 as default if required.

Change-Id: I30c77abef63636bb02db12d2f2b2d79ea244b96c
diff --git a/bsc/BSC_Tests.ttcn b/bsc/BSC_Tests.ttcn
index 7f670d2..34ee429 100644
--- a/bsc/BSC_Tests.ttcn
+++ b/bsc/BSC_Tests.ttcn
@@ -611,6 +611,8 @@
 	boolean mp_enable_osmux_test := true;
 	/* Value set in osmo-bsc.cfg "ms max power" */
 	uint8_t mp_exp_ms_power_level := 7;
+
+	boolean mp_media_mgw_offer_ipv6 := true;
 }
 
 private function f_gen_test_hdlr_pars(integer bssap_idx := 0) return TestHdlrParams {
@@ -623,6 +625,7 @@
 	}
 	pars.exp_ms_power_level := mp_exp_ms_power_level;
 	pars.mscpool.bssap_idx := bssap_idx;
+	pars.media_mgw_offer_ipv6 := mp_media_mgw_offer_ipv6;
 
 	return pars;
 }