Allow tasks to be scheduled on a specific cluster (#555)

* Allows schedule by cluster

* Update docs

* Add example in doc

* Fixes cluster field must be blank

* Adds cluster arg to schedule func

* Adds cluster field in list_display/filter for admin

* Uses Q filter to fetch schedule

* Fixes ref in doc

* Adds tests

django_q/models.py

@@ -200,6 +200,7 @@ class Schedule(models.Model):
         help_text=_("Cron expression"),
     )
     task = models.CharField(max_length=100, null=True, editable=False)
+    cluster = models.CharField(max_length=100, default=None, null=True, blank=True)
 
     def success(self):
         if self.task and Task.objects.filter(id=self.task):