enb_srs: expose DL AWGN channel modifier for srsENB

we've added the modifier to configure Amarisoft eNB channel
model. This patch enables it for srsENB.

Change-Id: I30e65d0431b2d2792986128287caf8b23a22b2c1
diff --git a/src/osmo_gsm_tester/obj/enb_srs.py b/src/osmo_gsm_tester/obj/enb_srs.py
index 7556f92..68a3a19 100644
--- a/src/osmo_gsm_tester/obj/enb_srs.py
+++ b/src/osmo_gsm_tester/obj/enb_srs.py
@@ -194,6 +194,8 @@
         self.enable_pcap = util.str2bool(values['enb'].get('enable_pcap', 'false'))
         config.overlay(values, dict(enb={'enable_pcap': self.enable_pcap}))
 
+        config.overlay(values, dict(enb={'enable_dl_awgn': util.str2bool(values['enb'].get('enable_dl_awgn', 'false'))}))
+
         self._additional_args = []
         for add_args in values['enb'].get('additional_args', []):
             self._additional_args += add_args.split()