*: make dial timeout configurable

Dial timeout is set shorter because
1. etcd is supposed to work in good environment, and the new value is long
enough
2. shorter dial timeout makes dial fail faster, which is good for
performance

integration/cluster_test.go

@@ -623,7 +623,7 @@ func mustNewHTTPClient(t *testing.T, eps []string) client.Client {
 }
 
 func mustNewTransport(t *testing.T) *http.Transport {
-	tr, err := transport.NewTimeoutTransport(transport.TLSInfo{}, rafthttp.ConnReadTimeout, rafthttp.ConnWriteTimeout)
+	tr, err := transport.NewTimeoutTransport(transport.TLSInfo{}, rafthttp.DialTimeout, rafthttp.ConnReadTimeout, rafthttp.ConnWriteTimeout)
 	if err != nil {
 		t.Fatal(err)
 	}