Thermos Health Check fails for MesosContainerizer if `--nosetuid-health-checks` is set

With MesosContainerizer, the sandbox is of type FileSystemImageSandbox and the health check is performed using a "mesos-containerizer launch" process, but there is actually a code bug in the way of getting the user under which to run the health check process:

https://github.com/apache/aurora/blob/master/src/main/python/apache/aurora/executor/common/health_checker.py#L370

health_check_user = (os.getusername() if self._nosetuid_health_checks
            else assigned_task.task.job.role)

If the Aurora scheduler is configured with `--nosetuid-health-checks` then "os.getusername()" is executed, but the python "os" module does not present a "getusername()" function.