etcdhttp: remove configurable timeout

It's slightly unclear why we expose this timeout as being configurable,
and the `-timeout` flag does not exist in 0.4.x, so for now, remove the
flag until we have evidence that it is needed.
Merge pull request #1215 from jonboulle/1215

Flag `-timeout` not descriptive
etcdserver: restore test coverage of cluster
Merge pull request #1238 from jonboulle/etcdserver_cluster

etcdserver: restore test coverage of cluster
raft: fix send
send should not attach current term to msgProp. Send should simply do proxy for msgProp without
changing its term. msgProp has a special term 0, which indicates that it is a local message.
Merge pull request #1240 from coreos/fix_proposal

raft: fix send
*:discovery hook up
Documentation: introduce the initial 0.5 clustering guide

Introduce the concepts of static initial cluster configuration and
document the flag migrations.
main: 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
```