The pserver need to have server index for saving model on the
distributed file system. The server index comes from etcd if etcd is
used, or user can manually specify them. So we need
pserver.NewService() to take index as an argument. Since index could
come from etcd, it would be strange if pserver takes an index as
argument, at the same time get the index from etcd. so we will need to
decouple pserver with etcd.

I made a comment in WuYi's PR that this is not necessary, so WuYi
removed it. Turns out it's necessary after confirming with coreOS
developer.

Please refer to the change design doc for what in API have changed.