Fix creating subnet without gateway

If create_subnet() method is called with gateway=None explicity,
subnet should be created without gateway_ip specified.
To achieve that "gateway_ip=null" should be passed in json in
request's body to neutron server.
This was missing, so neutron-server allocated gateway_ip automatically.
Now gateway for such network will not be set as is expected.

Closes-Bug: #1769609

Change-Id: Ia9f0646a3cf371f82f2aa2dc22837249531d1ff5

neutron_tempest_plugin/api/base.py

@@ -431,6 +431,8 @@ class BaseNetworkTest(test.BaseTestCase):
                 ip_version=ip_version, cidr=cidr, mask_bits=mask_bits):
             if gateway is not None:
                 kwargs['gateway_ip'] = str(gateway or (subnet_cidr.ip + 1))
+            else:
+                kwargs['gateway_ip'] = None
             try:
                 body = client.create_subnet(
                     network_id=network['id'],