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.
i think using infra1 would be a better example since some people might still think in the old way(infra1 only needs to know infra0). this can help them.

```
etcd -name infra1 -advertise-peer-urls http://10.0.1.11:2379 \
-initial-cluster infra0=http://10.0.1.10:2379 -initial-cluster-state=new
etcd: infra1(self) not listed in the initial cluster config
exit 1
```