Use MESOS_HTTP rather than HTTP for health checks

[bergerx]

Health checks in most frameworks (if not all) is still using HTTP rather than MESOS_HTTP, we experienced problems with some already at scale. HTTP is meant to be depreceated, from MArathon 1.4.0 changelog (https://github.com/mesosphere/marathon/releases/tag/v1.4.0):

We strongly recommend Mesos-based health checks over Marathon-based health checks. Marathon-based health checks are deprecated and will be removed in a future version.

At least the getting started guide should be updated soon to lead the new frameworks to use MESOS_HTTP.