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.
Sorry, my change snuck in so this is going to need to be reworked a bit