First draft of configuration proposal.

@smarterclayton 
Starting point for discussion. Feedback welcome. Also let me know where you'd like more detail.
What about having a user provided initial value, and a user readable but not writeable current value.
Like:
```
desiredState:
{ "type": "replicationController",
  "initial_replicas": 3,
  // user cannot set "replicas", or it is ignored.
}

currentState:
{ "type": "replicationController",
  "initial_replicas": 3,
  "replicas": 4 // set by auto-scaler
}
```