etcdserver: support newly-join member bootstrap

```
1 > $ goreman -f="Procfile" start
2 > $ curl -XPOST http://127.0.0.1:4001/v2/admin/members/ -H "Content-Type:application/json" -d '{"PeerURLs":["http://127.0.0.1:7004"]}'
2 > $ ./bin/etcd -name node4 -listen-client-urls http://127.0.0.1:4004 -advertise-client-urls http://127.0.0.1:4004 -listen-peer-urls http://127.0.0.1:7004 -advertise-peer-urls http://127.0.0.1:7004 -initial-cluster 'node1=http://localhost:7001,node2=http://localhost:7002,node3=http://localhost:7003,node4=http://127.0.0.1:7004' -initial-cluster-state existing
```
The cluster runs well.
it just wants to update ID, but not attributes.
New member has no name when POST, so if we use membs directly, we cannot find out which one is self..