Add test_update_plugin()
Add test_update_plugin()

There are get_plugin() and update_plugin() in service/api/v10.py,
but just test get_plugin,so I add test_update_plugin().

Change-Id: I10055fd3716c69e7681eb23651fb967f6ff15ff0

egafford/sahara,openstack/sahara,tellesnobrega/sahara,openstack/sahara,egafford/sahara,tellesnobrega/sahara