CeleryPingHealthCheck: Add 'limit' argument for ping

[cbwchuck]

CeleryPingHealthCheck makes use of celery.app.control.ping with timeout argument set.

But celery always wait for timeout seconds before returning the responses, which is a bit counter-intuitive.

If what we want here is to return the ping result immediately after receiving the responses, we will need to specify limit=<num_of_celery_workers_expect_to_see> as documented in Celery's documentation.