Make use of user-defined settings for defining the isolated nodes heartbeat

Just in case the user has created custom settings files which set
AWX_ISOLATED_PERIODIC_CHECK.

awx/settings/production.py

@@ -107,3 +107,12 @@ except IOError:
             raise ImproperlyConfigured(msg)
     else:
         raise
+
+
+CELERYBEAT_SCHEDULE.update({  # noqa
+    'isolated_heartbeat': {
+        'task': 'awx.main.tasks.awx_isolated_heartbeat',
+        'schedule': timedelta(seconds=AWX_ISOLATED_PERIODIC_CHECK),  # noqa
+        'options': {'expires': AWX_ISOLATED_PERIODIC_CHECK * 2},  # noqa
+    }
+})