os_server_action: pass imageRef to rebuild

Prior to this change, the os_server_action module could not rebuild
servers, because it passed no arguments to the "rebuild" object.

The imageRef parameter is mandatory. If the client does not specify
imageRef, Nova takes no action to rebuild the server.

Change-Id: I0edbdb634feb90d4c54476a4089fcb014c0b83b2

plugins/modules/os_server_action.py

@@ -233,7 +233,7 @@ def main():
             # rebuild doesn't set a state, just do it
             cloud.compute.post(
                 _action_url(server.id),
-                json={'rebuild': None})
+                json={'rebuild': {'imageRef': image.id}})
             if wait:
                 _wait(timeout, cloud, server, action, module, sdk)
             module.exit_json(changed=True)