Allow force stop server in nova rest api

For libvirt dirver, It has already implemented how to stop a server(gracefully or hard), and which way used is controlled by "clean_shutdown" flag in nova.compute.api.API.stop(self, context, instance, do_cast=True,clean_shutdown=True) method.
And this method is called by nova stop sever rest api, while this rest api does't support to deal with this flag(it always set the "clean_shutdown" flag as True), so I think we should add this support in rest api then end-user could choose which way to stop a server.