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
1 file changed