Zero's /health endpoint returns just "Ok" where the same Alpha's endpoint returns a JSON

I'm testing Dgraph 1.2.3 version and notice there is a difference in `/health` endpoints for Zero and Alpha. It would be good to unify that endpoint to return the same JSON structure.

Zero's response to `curl http://localhost:6080/health`
```
Ok
```

Alpha's reponse to `curl http://localhost:8080/health`:
```
{"version":"v1.2.3","instance":"alpha","uptime":59131}
```