Always log when a lambda is invoked

This replaces the previous single-purpose log about a link test
with a more informative and generic one for all invocations.

app/celery/broadcast_message_tasks.py

@@ -251,6 +251,4 @@ def send_broadcast_provider_message(self, broadcast_event_id, provider):
 @notify_celery.task(name='trigger-link-test')
 def trigger_link_test(provider):
     identifier = str(uuid.uuid4())
-    message = f"Sending a link test to CBC proxy for provider {provider}. Identifier in payload is {identifier}"
-    current_app.logger.info(message)
     cbc_proxy_client.get_proxy(provider).send_link_test(identifier)

app/clients/cbc_proxy.py

@@ -118,6 +118,10 @@ class CBCProxyClientBase(ABC):
     def _invoke_lambda(self, lambda_name, payload):
         payload_bytes = bytes(json.dumps(payload), encoding='utf8')
         try:
+            current_app.logger.info(
+                f"Calling lambda {lambda_name} for link test with payload {payload}"
+            )
+
             result = self._lambda_client.invoke(
                 FunctionName=lambda_name,
                 InvocationType='RequestResponse',