Convert delete server mocks to requests_mock
Convert delete server mocks to requests_mock

Change-Id: I1615aa825dffbddd33e5effc014415945a694e76

stackforge/python-openstacksdk,stackforge/python-openstacksdk,openstack/python-openstacksdk,openstack-infra/shade,openstack-infra/shade,dtroyer/python-openstacksdk,dtroyer/python-openstacksdk,openstack/python-openstacksdk