Scheduler: First-fit algorithm throws exception for empty device list

```Scheduler.js:282``` : ```devices[0]``` is undefined

```javascript
tasks.forEach(function(task){
	var most_space = Object.keys(mapping).reduce(function(acc, id){
		return (mapping[id].available_memory > mapping[acc].available_memory) ? id : acc;
	}, devices[0].id); 
```
Error thrown:
```
     TypeError: Cannot read property 'id' of undefined
```