use initial-cluster and initial-cluster-state flags

In preperation for adding the ability to join a machine to an existing
cluster force the user to specify whether they expect this to me a new
cluster or an active one.

The error for not specifying the initial-cluster-state is:

```
etcd: initial cluster state unset and no wal found
```

Documentation: introduce the initial 0.5 clustering guide

Introduce the concepts of static initial cluster configuration and document
the flag migrations.
give the comment for it that `discovery/6c007a14875d53d9bf0ef5a6fc0257c817f0fb83` is used to save member infos, and `_config/size` inside is for bootstrap size  ?