Add unit tests for "server stop" command.
Add unit tests for "server stop" command.

Change-Id: Idfa82f7f3aee3824aedf8b551be7942659530457
Implements: blueprint osc-unit-test-framework-improvement

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