Implement etcd flavor for TopoServer.

@alainjobart 

Most of the integration tests are passing with --topo-server-flavor=etcd, except secure.py and zkocc.py. Note that you need to install etcd 0.4.x in your $PATH, and install the go-etcd client library (rerunning bootstrap should do it).

I haven't found a way to create an in-process fake etcd (like we have for fakezk), so I can't run the in-process topo tests yet.

Also I still need to make the explorer plugin for vtctld.