Ping Celery workers when starting up.

This makes debugging Celery/RabbitMQ/Redis issues much easier, as it
happens at application startup, rather than when we first create a Celery
task.

pillar/__init__.py

@@ -345,6 +345,9 @@ class PillarServer(Eve):
             result_expires=3600,
         )
 
+        self.log.info('Pinging Celery workers')
+        self.log.info('Response: %s', self.celery.control.ping())
+
     def register_static_file_endpoint(self, url_prefix, endpoint_name, static_folder):
         from pillar.web.staticfile import PillarStaticFile