Fixing role id serialization

Change-Id: I419523f238d17b90dfebfffdbe12eec19968255d
Signed-off-by: Eduardo Sousa <eduardo.sousa@canonical.com>
diff --git a/osm_nbi/admin_topics.py b/osm_nbi/admin_topics.py
index a737301..924ebdd 100644
--- a/osm_nbi/admin_topics.py
+++ b/osm_nbi/admin_topics.py
@@ -907,6 +907,8 @@
         """
         content_keys = list(content.keys())
 
+        content["_id"] = str(content["_id"])
+
         for key in content_keys:
             if ":" in key:
                 content[key.replace(":", ".")] = content[key]