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/migrations/0014_schedule_cluster.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.2.2 on 2021-05-11 05:59
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('django_q', '0013_task_attempt_count'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='schedule',
+            name='cluster',
+            field=models.CharField(blank=True, default=None, max_length=100, null=True),
+        ),
+    ]