Support --discovery-srv like ETCD to form a new cluster

## Feature Request

### Describe your feature request related problem

ETCD supports cluster discovery from scratch with --discovery-srv so the cluster init information can be stored in very high available DNS and easier for people to like changing the peer address or recovering cluster from existed data (when the address is changed).

### Describe the feature you'd like

Let's say that we have the SRV records:
```
$ dig +noall +answer SRV _pd-server._tcp.example.com
_pd-server._tcp.example.com. 300 IN SRV 0 0 2380 infra0.example.com.
_pd-server._tcp.example.com. 300 IN SRV 0 0 2380 infra1.example.com.
_pd-server._tcp.example.com. 300 IN SRV 0 0 2380 infra2.example.com.
```

Start pd-server with command below will auto form the cluster from SRV records of example.com (user may have to define advertised-client-urls and so on...)
```
pd-server--name infra0 --discovery-srv example.com
```

### Describe alternatives you've considered

Prepare a true exact `--initial-cluster` option with proper `--name` option and `--advertise-peer-urls` while they are correlate.