Fix output for subnet show
Fix output for subnet show

The use_default_subnet_pool column is never returned by the API, it is
only being used in the request to create a subnet. So make sure that we
do not show it when displaying a subnet.

Change-Id: Ie021149cceb8f89b779ad0f3c13ac60420509671
Related-Bug: 1668223

dtroyer/python-openstackclient,dtroyer/python-openstackclient,openstack/python-openstackclient,openstack/python-openstackclient