Fix user group management in gerrit

Change-Id: Ic5ce0891b0fb292c11c314670817b3771fc33c26
Related-prod: #PROD-29665 (PROD:29665)
diff --git a/_modules/gerrit.py b/_modules/gerrit.py
index db2a118..e33013b 100644
--- a/_modules/gerrit.py
+++ b/_modules/gerrit.py
@@ -222,7 +222,7 @@
     logging.info('Creating membership of %s in group %s', account_id, group_id)
 #    group_id = _group_name2id(gerrit, group_id)
     path = 'groups/%s/members/%s' % (_quote(group_id), account_id)
-    gerrit.put(path, data=json.dumps({}))
+    gerrit.put(path)
 
 
 def _ensure_only_member_of_these_groups(gerrit, account_id, salt_groups):