tools/etcd-agent: stop etcd only if it is running

Stop etcd only if it is running, and not report error when stopping etcd
which is not started.

tools/functional-tester/etcd-agent/agent.go

@@ -73,6 +73,9 @@ func (a *Agent) start(args ...string) error {
 
 // stop stops the existing etcd process the agent started.
 func (a *Agent) stop() error {
+	if a.state != stateStarted {
+		return nil
+	}
 	err := a.cmd.Process.Kill()
 	if err != nil {
 		return err