Readonly proxy mode

This is rebased on top of https://github.com/coreos/etcd/pull/1106
My original concept of "proxy mode" included an option to attempt to join the raft cluster before falling back to proxying. That's where the "fallback" mode came from. If that were the case, the options for "-proxy" would be "on", "fallback", and "off". I separated out `-proxy-readonly` since that feels like its configuring how the binary does its proxying, not whether it is going to proxy or not. Thinking about it more, it doesn't really make sense to fallback to a readonly mode from trying to join the cluster.