Add multiple servers test case to TestServerDelete.
Add multiple servers test case to TestServerDelete.

This patch adds a multiple servers test case to class TestServerDelete.

Change-Id: I8b0c37ec2a8e7b23889e25c9004f867f2907f630
Implements: blueprint osc-unit-test-framework-improvement

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