etcdserver: separate machines with space

etcd v0.4.6

```
$ curl -i localhost:4001/v2/machines
HTTP/1.1 200 OK
Date: Tue, 26 Aug 2014 01:18:34 GMT
Content-Type: text/plain; charset=utf-8
Transfer-Encoding: chunked

http://172.17.8.101:4001, http://172.17.8.102:4001, http://172.17.8.103:4001


$ etcdctl --debug set /foo bar
Cluster-Peers: http://172.17.8.101:4001 http://172.17.8.102:4001 http://172.17.8.103:4001
Curl-Example: curl -X PUT http://172.17.8.101:4001/v2/keys/foo -d value=bar
bar
```

etcd v0.5

```
 $ curl -i localhost:4001/v2/machines
HTTP/1.1 200 OK
Date: Tue, 26 Aug 2014 01:19:28 GMT
Content-Type: text/plain; charset=utf-8
Transfer-Encoding: chunked

http://10.9.151.250:4001,http://10.73.129.121:4001,http://10.73.148.121:4001


$ etcdctl --debug set /foo bar
Cluster-Peers: http://10.9.151.250:4001,http://10.73.129.121:4001,http://10.73.148.121:4001
Curl-Example: curl -X PUT http://10.9.151.250:4001,http://10.73.129.121:4001,http://10.73.148.121:4001/v2/keys/foo -d value=bar
Curl-Example: curl -X PUT http://10.9.151.250:4001,http://10.73.129.121:4001,http://10.73.148.121:4001/v2/keys/foo -d value=bar
Error:  501: All the given peers are not reachable (Tried to connect to each peer twice and failed) [0]
```