Auto 877 set desired capacity in state (ready for review)

This does not write the desired capacity to the DB, since the state object marshaller/unmarshaller does not handle 'desired'.  That way this does not depend upon a [schema change](https://github.com/rackerlabs/otter/pull/541)

However, that is the only thing that should be added to start saving the desired capacity to Cassandra, which would be dependent upon [the schema change](https://github.com/rackerlabs/otter/pull/541).