celery: add task for triggering link tests

We want to periodically kick off some link tests, so that:

- we are periodically communicating with the CBC Proxies
- each CBC Proxy is periodically communicating with its CBC

This simulation of traffic to the CBC will give us advance warning if
something is going to break, or is broken, before someone tries to send
a real live message

Signed-off-by: Toby Lorne <toby.lornewelch-richards@digital.cabinet-office.gov.uk>
Co-authored-by: Richard <richard.baker@digital.cabinet-office.gov.uk>
Co-authored-by: Pea <pea.tyczynska@digital.cabinet-office.gov.uk>

app/config.py

@@ -308,6 +308,11 @@ class Config(object):
             'schedule': timedelta(minutes=5),
             'options': {'queue': QueueNames.PERIODIC}
         },
+        'trigger-link-tests': {
+            'task': 'trigger-link-tests',
+            'schedule': timedelta(minutes=5),
+            'options': {'queue': QueueNames.PERIODIC}
+        },
     }
     CELERY_QUEUES = []