API to indicate when all component instances are launched

Currently, we are querying the Slider AppMaster's "componentinstancedata" API in order to get the list of containers/hosts/ports of components launched by Slider.

When launching a large number of components, this API can report back with an incomplete list of containers/hosts/ports. Currently, we have to compare this list with the number of requested components.

Ideally, we would like to have some indication of "all components started" (either in a separate API, or as part of this existing api).