etcdserver: refactor cluster and clusterStore

Integrate clusterStore into cluster, and let cluster become the source of
cluster info.

fixes #1354 

early feedback welcome
@xiangli-cmu @philips The other idea that came to mind (though I'm not sure it's a great one so I didn't propose it) was to just add a (possibly optional) prefix to the `initial-bootstrap` string: 
`cluster1:default=http://localhost:2380,default=localhost:7001`
`cluster2:server1=http://localhost:7001,server2=localhost:7002`