tempest: Make _create_router_with_client obey enable_snat=False

Found while working on I4452d61ee6e9d21add3d37ec39301efcbd3bd66d

Closes-Bug: #1699006
Change-Id: Ideb3e57b068dce150e333fde66f7ad488c2b515d
diff --git a/neutron/tests/tempest/api/base.py b/neutron/tests/tempest/api/base.py
index 8213277..46b2849 100644
--- a/neutron/tests/tempest/api/base.py
+++ b/neutron/tests/tempest/api/base.py
@@ -328,7 +328,7 @@
         ext_gw_info = {}
         if external_network_id:
             ext_gw_info['network_id'] = external_network_id
-        if enable_snat:
+        if enable_snat is not None:
             ext_gw_info['enable_snat'] = enable_snat
         body = client.create_router(
             router_name, external_gateway_info=ext_gw_info,