Add task to send canary to cbc proxy

Create and schedule a Celery task that tests if we
can send a canary message to cbc proxy.

This will help us know if something happens to our
connection to cbc proxy.

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

app/config.py

@@ -303,6 +303,11 @@ class Config(object):
             'schedule': crontab(hour=23, minute=00),
             'options': {'queue': QueueNames.PERIODIC}
         },
+        'send-canary-to-cbc-proxy': {
+            'task': 'send-canary-to-cbc-proxy',
+            'schedule': timedelta(minutes=5),
+            'options': {'queue': QueueNames.PERIODIC}
+        },
     }
     CELERY_QUEUES = []