fenzo-core/src/main/java/com/netflix/fenzo/plugins/ExclusiveHostConstraint.java
@Override
    public Result evaluate(TaskRequest taskRequest, VirtualMachineCurrentState targetVM, TaskTrackerState taskTrackerState) {
        Collection<TaskRequest> runningTasks = targetVM.getRunningTasks();
        if(runningTasks!=null && !runningTasks.isEmpty())
            return new Result(false, "Already has " + runningTasks.size() + " tasks running on it");
        Collection<TaskAssignmentResult> tasksCurrentlyAssigned = targetVM.getTasksCurrentlyAssigned();
        if(tasksCurrentlyAssigned!=null && !tasksCurrentlyAssigned.isEmpty())
            return new Result(false, "Already has " + tasksCurrentlyAssigned.size() + " assigned on it");
        return new Result(true, "");
    }
@Override
    public Result evaluate(TaskRequest taskRequest, VirtualMachineCurrentState targetVM, TaskTrackerState taskTrackerState) {
        Collection<TaskRequest> runningTasks = targetVM.getRunningTasks();
        if(runningTasks!=null && !runningTasks.isEmpty())
            return new Result(false, "Already has " + runningTasks.size() + " tasks running on it");
        Collection<TaskAssignmentResult> tasksCurrentlyAssigned = targetVM.getTasksCurrentlyAssigned();
        if(tasksCurrentlyAssigned!=null && !tasksCurrentlyAssigned.isEmpty())
            return new Result(false, "Already has " + tasksCurrentlyAssigned.size() + " assigned on it");
        return new Result(true, "");
    }
Determines whether the prospective host already has tasks either running on it or assigned to be run on
it, and returns a false Result if either of those things is the case.

@param taskRequest      describes the task to be considered for assignment to the host
@param targetVM         describes the host to be considered for accepting the task
@param taskTrackerState describes the state of tasks assigned to or running on hosts throughout the
system
@return an unsuccessful Result if the target already has running tasks or assigned tasks, or a successful
Result otherwise