DHCP should be disabled on external subnet

External network should have dhcp option disabled. But the test
use base create_subnet option which have dhcp enabled by default.
We should pass enable_dhcp=False keyword argument explicitly to
create_subnet for external networks.

Change-Id: Ife13f8b0a890184d26bb941faec84dcc496c90b4
Closes-bug: #1420048
diff --git a/tempest/api/network/admin/test_external_network_extension.py b/tempest/api/network/admin/test_external_network_extension.py
index 06cce48..3110aab 100644
--- a/tempest/api/network/admin/test_external_network_extension.py
+++ b/tempest/api/network/admin/test_external_network_extension.py
@@ -96,7 +96,8 @@
         self.addCleanup(self._try_delete_resource,
                         client.delete_network,
                         external_network['id'])
-        subnet = self.create_subnet(external_network, client=client)
+        subnet = self.create_subnet(external_network, client=client,
+                                    enable_dhcp=False)
         body = client.create_floatingip(
             floating_network_id=external_network['id'])
         created_floating_ip = body['floatingip']