Introduce a general non-interpreting task check.

In addition to result-interpreting, killing health check, there is a requirement from Mesos framework authors for a general check that can execute an arbitrary command or send an HTTP request and pass the result to the scheduler without interpreting it.

This ticket aims to implement this functionality by introducing a new class of a check in Mesos. Design doc: https://docs.google.com/document/d/1VLdaH7i7UDT3_38aOlzTOtH7lwH-laB8dCwNzte0DkU