Implement gathering overall task capacity

For use when running/planning jobs

awx/main/models/ha.py

@@ -26,6 +26,10 @@ class Instance(models.Model):
     hostname = models.CharField(max_length=250, unique=True)
     created = models.DateTimeField(auto_now_add=True)
     modified = models.DateTimeField(auto_now=True)
+    capacity = models.PositiveIntegerField(
+        default=100,
+        editable=False,
+    )
 
     class Meta:
         app_label = 'main'