etcdctl members [list|add|remove]

This is fully functional (after #1459) lands, but it is not tested as well as it needs to be. I'm proposing this now as I want to get this in to make testing membership changes simpler for the rest of the team.
Any opinions on this? Looks like

```
42134f434382925: name=node1 peerURLs=http://127.0.0.1:7001 clientURLs=http://127.0.0.1:4001
5da88442b8f82f81: name= peerURLs=http://127.0.0.1:7004 clientURLs=
94088180e21eb87b: name=node2 peerURLs=http://127.0.0.1:7002 clientURLs=http://127.0.0.1:4002
9690ffae174d6fd9: name=node3 peerURLs=http://127.0.0.1:7003 clientURLs=http://127.0.0.1:4003
```

In this context, node `5da88442b8f82f81` was added through the API but has not yet joined and provided his metadata.