Allow creating subnet without a gateway

The ability to create a subnet without a gateway was broken
in commit 0ddc93b1b19922d08bedf331b57c363535bb357e.  Change
the create_subnet() method back to using '' as the default
value for the gateway so that callers can pass-in None as
before.  This allows the trunk test to revert to its previous
behavior of passing None.

Change-Id: Ib010f277f4c9ad8a708d16debdcdf8d2ceccd051
Closes-Bug: #1769609
diff --git a/neutron_tempest_plugin/scenario/test_trunk.py b/neutron_tempest_plugin/scenario/test_trunk.py
index 6fdcd5b..b5e8cda 100644
--- a/neutron_tempest_plugin/scenario/test_trunk.py
+++ b/neutron_tempest_plugin/scenario/test_trunk.py
@@ -230,7 +230,7 @@
         vlan_tag = 10
 
         vlan_network = self.create_network()
-        self.create_subnet(vlan_network)
+        self.create_subnet(vlan_network, gateway=None)
 
         servers = [
             self._create_server_with_port_and_subport(vlan_network, vlan_tag)