Deprecate --cluster-health

[danielmitterdorfer]

In the context of #293 we will establish a more generic solution than the current implicit cluster health check. With a future release of Rally (most likely 0.9.0 due to its breaking nature), Rally core will not perform this health check anymore implicitly but instead our standard tracks will add a new cluster health check operation explicitly.

Therefore, --cluster-health is not necessary anymore. It will instead be replaced by the new track parameter mechanism (see #310).

To ensure that Rally 0.8.0 can handle tracks with an explicit cluster-health check, we will introduce a new cluster-health call in scope of this ticket too. This will help with a smoother transition for users.