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.
ValidateAndAssignIDs validates the given members by matching their PeerURLs with the existing members in the cluster. If the validation succeeds, it assigns the IDs from the given members to the existing members in the cluster.  
If the validation fails, an error will be returned.

spelling and grammar checking? @jonboulle