Don't use non-existent webhook as a placeholder (#238)

The fake webhook fires the AlertmanagerNotificationsFailed alert
continuously out of the box. Instead, use a blackhole receiver without
any backend as a placeholder when no receiver is provided by an
operator.

Fixes: #237

src/config_builder.py

@@ -24,7 +24,7 @@ class ConfigError(Exception):
         "repeat_interval": "1h",
         "receiver": "placeholder",
     },
-    "receivers": [{"name": "placeholder", "webhook_configs": [{"url": "http://127.0.0.1:5001/"}]}],
+    "receivers": [{"name": "placeholder"}],
 }