etcdserver: specify timeout caused by leader election

Before this PR, the timeout caused by leader election returns:

```
14:45:37 etcd2 | 2015-08-12 14:45:37.786349 E | etcdhttp: got unexpected
response error (etcdserver: request timed out)
```

After this PR:

```
15:52:54 etcd1 | 2015-08-12 15:52:54.389523 E | etcdhttp: etcdserver:
request timed out, possibly due to leader down
```

Conflicts:
	etcdserver/raft.go

etcdserver/server_test.go

@@ -566,6 +566,7 @@ func TestDoProposalCancelled(t *testing.T) {
 
 func TestDoProposalTimeout(t *testing.T) {
 	srv := &EtcdServer{
+		cfg:      &ServerConfig{TickMs: 1},
 		r:        raftNode{Node: &nodeRecorder{}},
 		w:        &waitRecorder{},
 		reqIDGen: idutil.NewGenerator(0, time.Time{}),
@@ -1023,6 +1024,7 @@ func TestPublishStopped(t *testing.T) {
 func TestPublishRetry(t *testing.T) {
 	n := &nodeRecorder{}
 	srv := &EtcdServer{
+		cfg:      &ServerConfig{TickMs: 1},
 		r:        raftNode{Node: n},
 		w:        &waitRecorder{},
 		done:     make(chan struct{}),