Refactor TestCreateNetwork: Split TestCreateNetwork into two classes for identity v2 and v3
Refactor TestCreateNetwork: Split TestCreateNetwork into two classes for identity v2 and v3

In TestCreateNetwork, both Identity v2 and v3 clients are tested. As a result, we should
initialize the identity client again and again in each test function.

To reduce redundant code, this patch split TestCreateNetwork to TestCreateNetworkIdentityV2 and V3.
And then initialize the identity client only once in each of them.

Change-Id: I349fe7f827524beb541efe7dd9460c534254b80c
Implements: blueprint osc-network-unit-test-refactor
Related-to: blueprint neutron-client

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