Add section for search in cancellations

Into `get_document_by_id` in order to find documents in cancelation.robot

@@ -54,6 +54,10 @@ def get_document_by_id(data, doc_id):
             for document in complaint.get('documents', []):
                 if doc_id in document.get('title', ''):
                     return document
+    for cancellation in data.get('cancellations', []):
+        for document in cancellation.get('documents', []):
+            if doc_id in document.get('title', ''):
+                return document
     raise Exception('Document with id {} not found'.format(doc_id))