Add validation rule to fail if multiple Mesos healthchecks are defined.

Hi,

I have a Docker container and I want to add 2 COMMAND healthchecks for it:
1. curl -f -X GET http://$HOST:$PORT0
2. curl -f -X GET http://hostname-another-app:8095/status

First command passes. The second one fails.

Expected result:
Marathon should display the application as Unhealthy.

Actual Result:
Marathon displays the application as Healthy.

When I edit the configuration and leave only the second healthcheck (the one that fails), Marathon correctly displays the application as Unhealthy.

It seems that Marathon doesn't handle correctly multiple COMMAND healthchecks.