_spawn_metadata_agent function name should be _spawn_metadata_proxy

l3_agent.py:
def _spawn_metadata_agent(self, router_info):
        def callback(pid_file):
            return ['quantum-ns-metadata-proxy',
                    '--pid_file=%s' % pid_file,
                    '--router_id=%s' % router_info.router_id,
                    '--state_path=%s' % self.conf.state_path]

        pm = external_process.ProcessManager(
            self.conf,
            router_info.router_id,
            self.conf.root_helper,
            router_info.ns_name())
        pm.enable(callback)

    def _destroy_metadata_agent(self, router_info):
        pm = external_process.ProcessManager(
            self.conf,
            router_info.router_id,
            self.conf.root_helper,
            router_info.ns_name())
        pm.disable()