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.
@jonboulle we can do it in your PR.