Refactor TestDeleteNetwork: Use TestNetwork in TestDeleteNetwork
Refactor TestDeleteNetwork: Use TestNetwork in TestDeleteNetwork

Make TestDeleteNetwork inherit from TestNetwork, and have the same class architecture
as the other test classes.

Change-Id: I37047d7d13931b2a8f25665d6427efc05fc4f989
Implements: blueprint osc-network-unit-test-refactor
Related-to: blueprint neutron-client

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