work around for problem with Mesos offering semantic

When using Mesos with the fine-grained mode, a Spark job can run into a dead lock on low allocatable memory on Mesos slaves. As a work-around 32 MB (= Mesos MIN_MEM) are allocated for each task, to ensure Mesos making new offers after task completion.
From my perspective, it would be better to fix this problem in Mesos by dropping the constraint on memory for offers, but as temporary solu
See [[MESOS-1688] No offers if no memory is allocatable](https://issues.apache.org/jira/browse/MESOS-1688) for details for this problem.