Creating subnet for tagged network without GW.

In order to have only 1 GW passed to the VM we need to create
second subnet without GW configured.
This fixes the VM connectivity loss issue.

Change-Id: Iaea3cf0d10b814bcdc690df6192f9e729f77304f
closes-bug: 1685602
diff --git a/neutron/tests/tempest/scenario/test_trunk.py b/neutron/tests/tempest/scenario/test_trunk.py
index d5ffd45..3b49e39 100644
--- a/neutron/tests/tempest/scenario/test_trunk.py
+++ b/neutron/tests/tempest/scenario/test_trunk.py
@@ -227,7 +227,7 @@
         vlan_network = self.create_network()
         new_subnet_cidr = get_next_subnet(
             config.safe_get_config_value('network', 'project_network_cidr'))
-        self.create_subnet(vlan_network, cidr=new_subnet_cidr)
+        self.create_subnet(vlan_network, gateway=None, cidr=new_subnet_cidr)
 
         servers = [
             self._create_server_with_port_and_subport(vlan_network, vlan_tag)