Add parameter public_net in ServerSignalIntegrationTest

Closes-Bug: #1756810

Change-Id: I6bb8cdbe1814d151f3f20f98c19cfe21503a48f0
diff --git a/heat_tempest_plugin/tests/scenario/test_server_signal.py b/heat_tempest_plugin/tests/scenario/test_server_signal.py
index a43c74a..1823087 100644
--- a/heat_tempest_plugin/tests/scenario/test_server_signal.py
+++ b/heat_tempest_plugin/tests/scenario/test_server_signal.py
@@ -29,6 +29,7 @@
             'key_name': self.keypair_name,
             'flavor': flavor,
             'image': image,
+            'public_net': self.conf.floating_network_name,
             'timeout': self.conf.build_timeout,
             'user_data_format': user_data_format
         }