[SPARK-2204] Launch tasks on the proper executors in mesos fine-grained mode

The scheduler for Mesos in fine-grained mode launches tasks on the wrong executors. `MesosSchedulerBackend.resourceOffers(SchedulerDriver, List[Offer])` is assuming that `TaskSchedulerImpl.resourceOffers(Seq[WorkerOffer])` is returning task lists in the same order as the offers it was passed, but in the current implementation `TaskSchedulerImpl.resourceOffers` shuffles the offers to avoid assigning the tasks always to the same executors. The result is that the tasks are launched on the wrong executors. The jobs are sometimes able to complete, but most of the time they fail. It seems that as soon as something goes wrong with a task for some reason Spark is not able to recover since it's mistaken as to where the tasks are actually running. Also, it seems that the more the cluster is under load the more likely the job is to fail because there's a higher probability that Spark is trying to launch a task on a slave that doesn't actually have enough resources, again because it's using the wrong offers.

The solution is to not assume that the order in which the tasks are returned is the same as the offers, and simply launch the tasks on the executor decided by `TaskSchedulerImpl.resourceOffers`. What I am not sure about is that I considered slaveId and executorId to be the same, which is true at least in my setup, but I don't know if that is always true.

I tested this on top of the 1.0.0 release and it seems to work fine on our cluster.
It seems that `executorId` and `slaveId` are synonyms:
```
          offerableWorkers += new WorkerOffer(
            offer.getSlaveId.getValue,
            offer.getHostname,
            getResource(offer.getResourcesList, "cpus").toInt)
```
```case class WorkerOffer(executorId: String, host: String, cores: Int)```

For the usage of indexWhere, I would assume that its cost is negligible in this case, but I will see if I can find a more optimal way to do it.