Fix bug 744

Change-Id: Ife72a5d1c995fd9e7ecb07e06d6a881f4a4ccb99
Signed-off-by: Eduardo Sousa <eduardo.sousa@canonical.com>
diff --git a/osm_nbi/engine.py b/osm_nbi/engine.py
index 2bf6169..f7bc012 100644
--- a/osm_nbi/engine.py
+++ b/osm_nbi/engine.py
@@ -138,7 +138,7 @@
 
                 for _, value in resources_to_operations["resources_to_operations"].items():
                     if value not in self.operations:
-                        self.operations += value
+                        self.operations += [value]
 
             if config["authentication"]["backend"] == "keystone":
                 self.map_from_topic_to_class["users"] = UserTopicAuth