Add endpoint to set broadcast service channel

app/service/service_broadcast_settings_schema.py

@@ -0,0 +1,10 @@
+service_broadcast_settings_schema = {
+    "$schema": "http://json-schema.org/draft-04/schema#",
+    "description": "Set a services broadcast settings",
+    "type": "object",
+    "title": "Set a services broadcast settings",
+    "properties": {
+        "broadcast_channel": {"enum": ["test", "severe"]}
+    },
+    "required": ["broadcast_channel"]
+}