Fix disable_gateway_ip for subnet

When creating a subnet with disable_gateway_ip, the value
was not passed into the creation which would lead into
a subnet being created with a gateway, and then on updates
it would have the gateway removed.

Change-Id: I816d4a4d09b2116c00cf868a590bd92dac4bfc5b

plugins/modules/subnet.py

@@ -382,6 +382,8 @@ class SubnetModule(OpenStackModule):
             params['allocation_pools'] = self.params['allocation_pools']
         params = self._add_extra_attrs(params)
         params = {k: v for k, v in params.items() if v is not None}
+        if self.params['disable_gateway_ip']:
+            params['gateway_ip'] = None
         return params
 
     def _build_updates(self, subnet, params):